To all of the guys talking about diametre of barrels, there have been thousands of threads on every airsoft forum about this. Consistency of barrel over diametre.
As for good brands, Mad bull is the best bang for the buck. I've never tried a Modify barrel but it should be pretty good but you can only use the rubber that goes with it. I'd highly recommend Prometheus, they're the only barrels I use, they're expensive but for a reason.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN