I think the spin-offs are some of my favorites from the franchise outside of the main games. There's something a bit more fun about them that satiates me.
Pokémon Puzzle League is my all-time favorite. Puzzle games are my weakness, and very addictive. I swear to you I suffered withdrawals when I realized I was playing it too much one year. My best memory of the game was defeating Mewtwo the first time I fought him in a minute-and-a-half. (It was Very Hard mode, but still, considering Gary's final battle always gave me a hard time, reaching Mewtwo was shocking and rewarding for me.)
Pokémon Pinball: Ruby & Sapphire was another addictive spin-off as next to puzzle games, pinball was another weakness of mine. If I had the original, I'd probably like that just as much, although the Ruby and Sapphire version was prettier to look at. I don't remember what happened to our copy, but I remember getting almost all of the Pokémon but one--I don't remember which, though, and I don't think we ever got it. Either way, I got the highest scores on both fields.
Pokémon Mystery Dungeon is probably the best the franchise has ever given us. I never thought I'd like the games as much as I did since I've never played a dungeon crawler prior to it. The characters and story were the most surprising if the games, I honestly never expected Pokémon to take a turn for the dark.
Pokémon Snap was a lot of fun. We sadly don't have our copy anymore (I don't remember what happened to it), but it was a game I played a lot even if I never fully understood how the pictures were scored. I wonder if I could do so much better with it nowadays if I could play it again.
Pokémon Stadium was nice since it was mostly my brothers and I who played it. When it wasn't Mario Party, Diddy Kong Racing, or Smash Brothers, we used Stadium to compete with each other. I believe we only beat the Gym Leader Castle once (that first time around, anyway), and we reached Mewtwo once (actually... twice now that we finally reached Round 2 a few years ago lol). The Kids Club was nice, and the GB Tower was an awesome feature. We never got Stadium 2 for some reason, although we had rented it from Blockbuster once.
I've seen Chuggaaconroy's let's play of Colosseum and that was enjoyable. I regret overlooking it as a kid, and I'm sad I may never get the chance to play it for myself.