I think Diantha is okay. She's not my favorite champion (Steven. <3) but she's alright. I liked how she didn't play a major role in the story, because it surprised me when I finished battling the E4 and there was Diantha.
I haven't actually gotten to the E4 in any of the other games besides AS and X, but from some knowledge from Adventures my favorites would be Steven, Cynthia, Lance, and Red.
I blame Fantina.