G&G - I tried one bag of G&G... All I have to say is never again. Shot like shit.
Golden Ball - I was given a sample bag of their .3s once and they were horrible. Accuracy was not horrible but was definitely not good. They did not feed properly in my PMags as EPMs, and the surface looked almost powdery so I was not surprised.
MadBull - I used them when I was a noob, but I've since stopped using them. They do not shoot consistently in my setups and I had lots of fliers curving in random directions. Feeding at high speeds was an issue.
Elite Force - I used these for a while and had respectable results, no complaints. Shot straight, fed fine even at extremely high speeds.
Valken - I got a sample of their .3s and I had no issues. Shot respectably, no feeding issues at high speeds.
BB Bastard - These are what I use exclusively now. No feeding issues, and more importantly my guns are shooting more accurately than when I used Elite Force, although its possible that my guns have gotten better. My only complaint is that the bags are horrible. They always break, but if you report the bag failure to them they'll take care of you.