Does Gotou count? I liked Gotou's whole arc to becoming Birth but since he actually becomes a rider that's probably disqualified...

I guess I'd have to go with Natsumi from Decade even though yes, she ends up becoming a Rider too.

Ozawa from Agito, Inoue's last good series before his [divorce/ alcoholism / WoW addiction] severly crippled his abilities henshining him into the infamous INOUE ZONE!

Goro from Ryuki - in my opinion, the first gay guy in Rider history. I mean, c'mon!

Reiko Momoi from Ryuki - she's just awesome.

Most annoying:
"TAKUN!! ORPHENOCHUU!!"
Onari
Kivat
Well, nearly everyone in Kiva.

Ankh and Ichijou.

Kengo
Ankh
Asuna

Three way tie.

Well, to add more fuel to your fire that is hate towards Keitaro, don't forget that he had to take a metaphorical dookie on all things precious for his moment of glory in the Faiz Movie Paradise Lost by drinking a homebrew Red Bull of sorts aka the one-shot henshin drink and becoming Kaixa to defend the resistance base against a tough orphenoch, which he succeeded, but the Kaixa belt withered away like a vampire under sunlight. :lolol

Alain from Ghost. Everything about him was awesome.

(Granted I've only seen a couple series, I might still discover somebody I like better)

As Bugs Bunny would say, "Wotta Maroon!"

I have only seen W, Gaim and Ex-Aid, but I just want to say that I'm a big Narumi Akiko fan.

It's funny because the character seemed extremely annoying at first, but turns out to be... Well, still pretty annoying, but in an endearing way (the slipper gags never get old for me!). She's a very brave and resourceful detective-in-training.