Strongest I've felt so far is the very latest gen AR from KWA, but I'm not sure if that's even out in the US yet. The GBLS DAS is also good, just mega bucks.
There's also the American Airsoft Factory gun, but I've never handled one. TM doesn't rattle much, but it's best catered for in terms of availability of the guns and of parts as well as any aftermarket changes you might want to make.