(I don't consider XD, Colosseum, the Stadiums and PBR Spin-offs, so they're off the list.)
My favourite spin-offs are TCG (GB), Snap and Ranch.
TCG because it simply is a portable, single player version of the card game I find amusing.
Snap just because I used to play it a lot as a kid...
And Ranch. It itself is not good, but I can store my useless eggs there without the need to hatch them and then release. (I have about ten full boxes of eggs there...)
Ranger 1 was the only ranger game I played, and it was okay too.