I've been wanting to do this for a while! Now, I suck at narrowing them down and making top ten lists, so instead I'm going to be listing them chronologically starting with...
Battle Fever J! Now, Battle Fever J is a hit and miss with some people, but I personally enjoy it, and one of the reasons I love it is because of the costumes! They are extremely unique, and, sadly, there hasn't been outfits like that since. I love the detail put into each costume, making them unique instead of totally uniformed. This is when Marvel was working with Toei, I believe, and you can see the superhero influence on them. I like how they show off each country, and a little bit of each culture. And for those of you who think they look silly, the characters own the costumes, which makes them even more formidable.
Dynaman. Denjiman was the first step in the right direction helmet wise, and Dynaman was the first step in the right direction suit wise. Previously the outfits were made out of cotton, but this was the first series to use spandex (although BFJ might have used it, I think), and to forgo the fashion nightmare that was the effing ascots! Seriously, what the hell was with the ascots? Ascots aren't cool, they're one of the worst parts of late 70s fashion, and apparantly one of the costume designers was like "1977 called, they want their lame ascots back!" I think this (the ascot) was one of Shotaro Ishimori's contributions, but, and I'm gonna sound sacrireligious, they looked really, really, really stupid! So no ascot, but not only that, they started using spandex. The spandex was not only much more flattering, but also gave the suit actors more economy to move around in. And honestly, they looked a lot more comfortable fighting baddies and not having to worry about a rip.
Bioman. Bioman's here because I like the unique helmet designs, along with how the microchip-like chest plate lit up after the transformation sequence. The helmets were truly one of a kind, and I kind of wish a series would use that kind of design again.
Flashman. I like Flashman mainly because of the simplicity of the outfits, but also because of the function of the helmets, how the visors (or "goggles") actually can retract (or "set"), which they actually used in one of the earlier episodes, putting masks over their eyes so their vision wouldn't be screwed up by the Monster of the Week. It's one of the series where they actually took advantage of their suit's design. It was also nice during the transformation sequence that they used the "set goggle" part, mainly so that you can see the actor's eyes behind the helmet.
Maskman. In addition to the unique helmet designs, Maskman was the first series where I can say that the outfits were actually very sleek and stealthy. And it's the only series where you kind of forget the girls have earring on their helmets mainly because of how cool the suits look.
Liveman. Although a series before it (Sun Vulcan) had an animal theme with the suits, Liveman was the first series to truly take advantage of it. The animal of choice was incorporated into both the suits (with an animal patch on the chest), and with the helmets. Sun Vulcan had a small bust of the animal on the top, over the visor, but Liveman was like, let's make the whole helmet look like the animal. This lead to some extremely unique, creative, customized, and beautiful helmets, my personal favorite being Yellow Lion's. In addition, Red Falcon's helmet design would be used again in a slightly different variation for...
Jetman. This series used the bird design of Red Falcon's helmet, customizing it to each bird, and making it slightly sleeker and rounder. If you look closely, each Jetman's helmet is made to look like the bird they represent. It's also the sentai version of one of Jetman's inspiration series, Gatchaman. My personal favorite part, and the reason they're on the list is because of the wings that can be retracted, and how they use them to fly.
Zyuuranger. I think of the reasons MMPR became iconic, and really stuck with the fans was because of the suits. The helmets once again used the Liveman formula of having the helmet decked out to represent the animal or dinosaur that the Zyuuranger represented. And they are so intricate and ornate. They also used the thing where they put lips in the mouthpiece, and I think the last series to use that was Bioman. This made the helmets look a bit more human-like. They also gave the sixth ranger something a bit more special, the golden vest, setting him apart from the other members of the team, showing an outsider status. But not only that, his vest actually did something, it protected him. Once again, major points for parts of the suit having a function outside of just looking cool.
Kakuranger. I personally love the minimalism of the Kakuranger's outifts and helmets. Why? Because they're ninjas! Minimalism is everything to ninjas! The outfits actually look like the uniforms (is uniform the right word?) that ninjas would wear, with a sentai spin (the helmet looks like a ninja hood, with a visor where the open part for the eyes would be). Along with the regular ninja uniforms that the team wears, this makes their sentai forms look like an armored version of their ninja outfits.
Carranger. Carranger was a very silly series, so let's give the Carrangers a very silly outift. It was one of the sentai series to use an outfit to match the tone of the series. Also, I like how they incorporate the Carranger's cars into the helmet design.
Megaranger. I like the sleekness and the uniformity of these suits, which is perfect, as the whole Megaranger concept is technology, and the suits were probably designed to be technologically advanced. When you go back to another sentai series that incorporated a little bit of technology (Bioman), the suits were bulky compared to Megaranger, which shows how when technology evolves, technology gets sleeker. (Sorry I didn't use a pic with Megasilver, but I find this picture to be too cute)
Gingaman. Once again, amazing helmet designs. I like how the visors are shaped differently as well for the team members. They don't fully look like their animal, it's more like their helmets are symbols of their animal instead, which kind of goes with the kind of tribal (is that the right word?) aspect of the Gingaman.
Gaoranger. Once again, they go the Liveman route of having the whole helmet look like the animal they represent. In addition, they actually have claws on the gloves. As well, there are some subtleties to the costumes. On the chest of the suit, there's like this diagonal sash with gold lines, which go hand-in-hand with the number they press on their transformation device. (Gaowhite presses one, Black two, Blue three, Yellow four, Red five), so white has one slash, blue has two, and so on. Also, they took a cue from Jetman's design and gave GaoYellow a set of wings, only instead of the cape-like wings of Jetman, these looked like actual feathers. It's pretty rare that the Yellow Ranger has an upgrade on the suit that sets him apart from the rest of the team--this usually is given to the sixth ranger. But it kind of goes with Gao Yellow's own character (he's more serious, resentful of red, and the lancer which makes him kind of an outsider).
Hurricanger. In addition to having a unique suit design, the team takes a cue from Flashman, and has the helmets actually do something--they can open up, showing you there is a hero behind the mask. And they even gave Shurikenger's helmet something to do as well.
Abaranger. Abaranger is like a sleeker, more simplictic version of the dinosaur motifs from Zyuuranger. But what really puts Abaranger on the list is the super mode. It takes the spikey triangles on the suit, and has them stick up, like an actual dinosaur's spikes. Instead of other series where the super mode is usually another layer of armor (or usually comes mid-series with an upgrade), Abaranger simply has part of the suit stick up, and become 3-d. In addition, major props for giving yellow wings like her Pterodactyl.
Dekaranger. Taking a cue from Megaranger, Dekaranger once again has a sleek, advanced, functional uniform. It's one of the few that has the girls on the team not wearing skirts or a variation (Bioman, Changeman, and Flashman didn't have skirts but the bottoms were different on the female members). The last time the suits were like this was with Fiveman. This gives the team a kind of uniformity that you'd expect from the police. As well, the SWAT mode might have been armor, but it fit the motif well, it looked like stuff you'd wear if you were doing SWAT.
Gekiranger. Gekiranger once again uses the animal-based helmet designs of Liveman, but instead makes them more simplistic and sleeker. In addition, both GekiViolet and GekiChopper have more different yet suits, with kind of a martial arts motif (for example, GekiChopper's suit looks kind of like a gi). Gekiranger also has a unique super mode, where the suit changes colors, and get these cool jets. I also like how the main trio uses their own gloves as a transformation device.
Shinkenger. Previously, we've seen some more abstract, geometrical helmets with Ohranger, but Shinkenger kind of has a cooler sleekness to them. I like how the kanji bounces out from the helmet. There's kind of a more 3D element to it. There's this simplicity to it that works.
Gokaiger. They take the pirate motif and run with it. The helmets all look like captain's hats (or in Silver's case, a bandana), and it's the only suit to actually have jackets as part of it. As well, as a 35th aniversary series, there are a few callbacks to some of the past sentai series. The collars are reminiscent of Goranger, however, the suits make them their own as part of the pirate-y jacket. The helmet visors shapes are reminiscent of both Goranger, but also reminiscent of Flashman, another sentai team from space (although they are all earthlings).
Toqger. Say what you will, the suit designs are silly, but they are reminiscent of another silly series, Carranger. The helmets actually look like their own trains, and I like how Toq2go actually uses the train tracks like they're his glasses, and adjusts them as well.
Ninninger. Ninninger has probably the coolest suit designs in a while. Why is it that ninja sentais have cool suit designs? I like how, much like Shinkenger, the shurikens pop out. In addition, red has a asymetrical shuriken, compared to the others, because it's supposed to be moving.