To keep things simple, let's just define "spin-off" as a Metal Gear game that isn't part of the Kojima's Metal Gear Saga, so basically anything other than the MSX games, numbered Solid games, and Peace Walker.
Personally, I like Ghost Babel and Rising the most. The former is a really impressive GBC game that's also really fun and has amazing music, and Rising is just a really fun hack-and-slash with really good vocal songs.
Personally, I like Ghost Babel and Rising the most. The former is a really impressive GBC game that's also really fun and has amazing music, and Rising is just a really fun hack-and-slash with really good vocal songs.