Tough question.
Dunno that I've got ONE favorite, bit I've got favorites for specific uses....
FWB300s Mini for extended offhand close range/tiny target or paper shooting sessions, 10meter type stuff in the garage late at night or just messing around short range plinking with other airgunners. As for why: not as heavy as the full-size model (can shoot offhand longer without getting worn out), springer so no concerns about running out of or using air (can shoot for a LONG time, worry free). recoil free so pcp-like accuracy, 300s stocks are just a joy to hold and shoot. (4x UTG scope and just about any pellet, bout 6fpe)
MAC1 USFT for SERIOUS field target. (see my posts on the extended breakdown for the whys) (Falcon 10-50x and JSB 10.34 for 19fpe).
HW50s for 30-55 yard fun. Why? powerful enough to reach out there, but not too powerful to make it too easy, lots of technique required (recoiling springer), springer (high volume shooting). (SWFA 20x and JSB 7.87 for about 9fpe)
Veteran Long .22 for high (relatively) power, long range. Great trigger, consistent fps, ACCURATE (too powerful to make anything less than about 50 yards very fun or economical in air or pellets) (SWFA 20x and JSB 25.4gr for 42-44fpe)
Veteran Short in .20 is probably my favorite in the all-around, general use airgun. It can do all of the above pretty good, but not any of each specific use as good as the above guns. (Athlon Midas Tac and JSB 13.73gr for 19fpe or 15.89 for 29-30fpe).