SR2 is the best and one of my favorite games in general. The map was well designed and didn't feel like there was much filler, the customization was great, the soundtrack hit, gameplay was great, and lots of memorable missions.
SRTT may be the best for just going wild though. Sure, it is missing a lot of the quality of life aspects and parts of the map really feel cut-and-paste, but the gameplay feels really fluid and I think this is where the series hit the perfect balance between serious and wacky. I don't like how they tied activities to territory especially since they got rid of a lot of the funner activities like Fight Club and FUZZ.
SR1 is pretty much just a shittier version of SR2 in everything but story. Probably has the best story of the series actually, though that is hardly a strong suite for the series anyways.
SR4 just kinda jumped the shark for me. It is still fun, but feels more like Crackdown on ...crack.
Haven't gotten around to play GooH yet