Probably Alder's. No real bells and whistles, not intimidating, and while it's energetic, it doesn't have enough energy to make that work either. It's a nice little ditty, but not a great champion song.
Biggest disappointment for me was Hoenn, easily. I'm a big fan of brass and Hoenn has the most brassy soundtrack, and yet the champion theme was nothing special. And then the remake version has nothing brassy about it at all, and is still not particularly great.
Diantha is probably the song I changed my mind the most on- first time I heard it I thought it was underwhelming, but then I heard some people mention it sounded way better with headphones, and it really does. It's one of the better ones with them.
