Worst: tough one, there's my Cyma MP5 which never fired right from the day I got it, would work perfectly at home but get to site and it'd fire about once in every 3 or 4 trigger pulls, currently in pieces in a box. Then there was the WE Hi-Capa 5.2R, the one with the meat tenderiser on the front, loved the look of it but just couldn't get on with it in-game, sold to a mate who was really happy with it (odd bloke). There's also my SRC G36C (Gen III) I bought second hand from a mate for £80, boxed, with a load of mags, was pretty good up until the gearbox decided to eat itself.
Best: not any easier! There's my Silverback SRS 16" Sport with a few choice upgrades (thank you annual bonus!) which is just incredible. My WE G18C with the "fun switch", Mad Bull hop & Crazy Jet barrel, pretty much the range of an AEG. AEG wise, I adore my Krytac PDW MkII, light, compact and (with a Lonex inner barrel & Prommy purple) insane range and accuracy. Then there's my aforementioned G36C which went through a bit of an internal transformation after it broke, too many tweaks to list, I now run it on 0.3s, I can't wind a hi-cap fast enough to keep up with its rate of fire and it's literally like a laser (especially if I put the tracer on it), it's pretty much a one shot = one kill RIF now, and it's running at around 320fps on 0.2s.
I honestly don't know why but a lot of folk don't like to be on the opposing team to me??? 😈