There's a good handful for me.
Twisted Metal Small Brawl is a spinoff of the Twisted Metal series, aimed at a younger audience and with rc cars instead of full sized vehicles. It's a pretty solid entry, and is more a spinoff thematically than mechanically, though it is overall a bit easier, slower, and floatier than other games in the series, especially compared to the most recent games at the time. (It came out around the same time as Twisted Metal Black.)
Pokemon Ranger, particularly the second game, Shadows of Almia. I love the general sense of humor, and the gameplay loop (pun intended) is just something I find to be pretty fun. Just a very charming series.
Megaman Battle Network probably has not only the honor of being my overall favorite spinoff series, but probably having the biggest difference in terms of enjoyment between the spinoff and the original for me. If I hadn't gotten Battle Network 5 for Christmas one year, and seen the NT Warrior anime series around the same time, I'm not sure I'd have even given it the chance. But, after having put many hours into playing 5, I went back to play the others later, and I absolutely love the series.
Summon Night: Twin Age is the only game in the series I've played, and I was admittedly very disappointed to find out that it was a spinoff. It's a game that I enjoyed enough to want more, and as far as I understand it's pretty self-contained.
RageBurner
