I really liked Colosseum (and XD), and I'm surprised that more people haven't played them. They don't seem to be all that popular, but I really liked the snagging/shadow Pokemon element. I also liked the fact that Colosseum introduced Johto Pokemon again, which weren't accessible at that point post-gen2/gen3 reboot. I'm also a big fan of the Mystery Dungeon series, with the Explorers games being my favorites. And another somewhat underrated game that I like is the TCG game for GB. It's pretty quick to beat, but it's a fun change of place from the other games. I would really like to play the sequel sometime (the one that was only released in Japan), but I haven't looked into how to do that yet (presumably, I would need it translated).
I've never had any interest in the Ranger games, for whatever reason. Pinball was ok, and I played both of those, but wasn't that into them. The Stadium series is great, though frustrating at times when done with rentals (but satisfying to beat, and a lot of fun if importing a team from my main series games). I enjoyed Snap, though I wish it was longer and contained more Pokemon. Puzzle League was also fun, though not as compelling for me as games with more of an RPG plot.