I guess I'll go with Japanese openings first:
"Kaisei Josho Hallelujah"- The first opening theme for GX. It isn't a horrible song by any means, but I didn't feel a good beat with worRAB. The other opening themes were better, especially Tear Drop and Precious Times, Glory Days.
"Kizuna"- You can count me as another person who didn't like the first 5D's opening theme. I really couldn't get into this song at all. I agree that the beat before the singing was pretty good, but I just didn't think that the singing worked for it. Last Train is completely awesome though.
I can't remeraber any other Japanese openings that I really didn't like, so I'll go with the dubs.
Pokemon Diamond and Pearl- I love Pokemon and all, but I really hated listening to this song with every episode of Diamond and Pearl. I always thought that all of the Pokemon openings had a good beat and some really great lyrics. This song proved to me otherwise. It felt like they didn't put any effort into it at all. The only good part about this opening was the scene with Lucario in it. I'm sure that's really surprising to hear from me as well. At least they improved with Battle Dimension's theme.
I'm almost wanting to say the opening theme for GX, but I kind of liked it better in season two and three than I did during season one, possibly because Jaden wasn't saying "Get your game on," in nearly every episode, so I won't count that one. I can't think of anything else at the moment.