I'm not quite sure what answer to give.
I think D&D is the only rpg I play which uses a d20. Sure, I've played Pathfinder, but that's still basically just D&D.
Out of the various editions I'm familiar with, I'm not sure that I have a favorite. I have a love/hate relationship with both 3rd and 4th. They both manage to do things that I think are really good, but then they also both have things which really bug me. Out of the two; despite some of my problems with 4th, I might lean slightly toward favoring it due to feeling that the ideals behind encounter design are a little closer to my preferences for encounter design.
I'm not familiar enough with 1st or 2nd to feel like I have an informed opinion. Some of my favorite books to read and look at are 2nd Edition books though. I cannot explain why. There's just something about the layout, style, and writing which speaks to me.
I don't know how to describe my feelings toward 5th yet. I feel enthusiastic about it when I first start a campaign or an adventure, but it seems like I lose motivation quickly.
Anyway, being that D&D is -I believe- the only d20 game I play, my default answer has to be D&D.
I'm not sure that I play any games which use d10 as the core mechanic.
I play Edge of The Empire, and that uses a dice pool.
I play GURPS 4th Edition which uses 3d6.
I play D&D which uses a d20.