>>716524590
The first game is worth checking out if you want to see how the series started, it's decent. The X68000 port and Reboot improves on it.
Boomerang is a neat remix of 2 and is worth checking out if you like 2.
Panorama is a great rail shooter that essentially combines Cotton with Space Harrier, it also has a very surreal and whimsical setting.
Rock 'n Roll is the newest entry in the series (until High Tension comes out) and features a ton of different playable characters and levels for different scoring routes.

>>715534496
Cotton!