Gale of Darkness or Pokemon Snap. I actually really liked both of them. Kind of got annoyed at reviewers scoring Gale of Darkness low over not being a collectathon. Snap would need more gameplay added in my opinion, though. It's too short as it is.
Edit: Most of the spinoffs are okay, really. Pokemon Channel was a bit better than some gave it credit for, and so are the mystery dungeons. Haven't played Ranger, though. Anyways, my list for wanted sequels goes like this:
1. Pokemon XD: Gale of Darkness
2. Pokemon Snap
3. Pokemon Channel
4. Pokemon Pinball (the GBA one is alot of fun, actually. Get the same developer to make a sequel and I'd be happy.)
5. Pokemon Mystery Dungeon