I've owned a bunch.
There are multiple categories of "best":
Nicest to shoot/tune/tweak: ICD Cats for sure. Quite smooth, well made, unique design, unmatched when it comes to being able to adjust all parts of the firing cycle.
Most fun to shoot: Tippmann SMG/68 Special. Liquid CO2 and a heavy hammer with a long throw = ridiculousness. The massive CO2 cloud and the marker jumping with every shot due to actual recoil... Factor in full auto on the SMGs. Wow.
My favorite: Tippmann Pro/Carbine. The ergonomics are just perfect for me. Fairly good performance, built well, tough as nails.
Addressing some other names thrown around:
Spyder SE: Probably one of the best actual Spyders. Not saying much though. Nothing really that special (I own one). AKA Black Widow, BL Millenium, and AM Sentinel are all better.
Pro-Lite: I just don't care for the dual-pin valve. The CVX is just much cleaner and simpler. Performance is generally better (except maybe on liquid).
A-5: Anything with a plastic power tube can't be considered the best. I like the A-5 a lot. The cyclone is brilliant. But put it on a better base gun and you've got a winner.