[warning: long ass post]
I've been thinking about it, and I have to say these are my favorite designs throughout all of Kamen Rider:
Ichigo:
-its sleek, aged well like a fine wine, and pretty much is the bar for the design of Kamen Rider.
V3:
-a really great and colorful suit that was the first different rider suit after Ichigo and Nigo.
Amazon:
-first non cyborg based Rider suit and definitely has this fierce look to it, reflecting the wild man that is Amazon
Super 1:
-I dont know what it is that makes me love Super 1's design, but it's flashy yet sleek and the silver really stands out and I love it.
Black:
-sleek, powerful, the perfect blend of cyborg and organic that makes this suit work well for me. (just look at that those organic joints!)
ZO:
-it feels like a mix of so many different Rider designs, like V3's helmet, Amazon's stripes, Black's sleekness. it just really is one I like
Agito:
-I love the mystic yet empowering look to Agito. the gold silver and black with slight touches of red just give me the feeling of a warrior of magic and might (I do love magic based super-heroes) and his other forms in the series blend well with the design.
Faiz:
-the definition of a technological rider suit that is bult for both offense and defense, and it looks REALLY GOOD in the night scenes of the show with the yellow and red of the suit glowing bright.
Den-O Ax form:
-this is my favorite of the Den-O forms because it reminds me of Agito and just has this powerful but subdued look to it.
Diend:
-as much as I don't like Decade and the series that failed to deliver, Diend has a look that is just amazing and makes me wish he was the main character of decade because of how cool he is. Also his henshin device is his gun. but fuck his final form though. that thing is ugly.
Wizard:
-I love red and black and wizard has the best combination of my two favorite colors that I have seen in Kamen Rider ever. also, coat tassels are an appropriate substitute for scarfs, but can never replace them. also magic heroes again.
and then there's Gaim suits that I can't even begin to name, but I guess I have to say Baron's Mango Arms is my favorite.
JUST LOOK AT IT AND ALL OF IT'S GLORY!
it also helps that my favorite character in the show is Kaito.
hopefully that answers your question
@Cybermanworf sorry for the lengthy post, but I just can't choose a single suit design.
and it does seem like for those of us watching Gaim, we all have different favorite characters and why we like them. I love how Gaim has like, almost no bad characters!