It's a tie between Lance and Cynthia for me. Lance redefined the meaning of being a Dragon Master with a kickass theme to go with it, and Cynthia has one of the most frustrating parties in the Pokemon games and she destroyed me when I first battled her. I honestly can't decide between the two of them.
I love Steven! Hoenn is my favorite region and Steel-types are one of my favorite types, so he's such an easy pick. Plus I really enjoyed his role in ORAS, it was a cool switch-up since RSE. I was disappointed that he got replaced in Emerald :(
Cynthia is the best one. The only other two I liked were Steven and Iris, the rest were kinda like cardboard cutouts to me. Except for Adler I guess, but he didn't even function as a Champion until you went to rematch the E4.
I can't remember what I said a month back in the previous incarnation of this thread, but Cynthia easily. Other than your rival in Gen 1, she has the only team that could actually stand a chance of normally beating the gym leaders and Elite Four. And several other reasons...
Alder just felt like a "uh we need an idea for a Champion let's just stitch together some Champion-y stuff in someone with little personality" approach by Nintendo.
Diantha meh. Just meh. Wallace a bit better but still meh. I don't even remember Iris being champion when I finished W2 that is how prominent she was... No comment on Lance, never grew up with Gen 2.
I like the idea of having Gary (red/blue/whatever) as the Champion because he's your rival
but that kid is such a dirtbag I had to give it to Lance the Dragon Master for obvious reasons.
(Followed very closely by Iris, who was followed by Steven.)