Frames are rarely a weak point on bikes, esp mid-range MTBs. The usual point of failure is wheels. Most MTBs seem to have standardised on 32spoke wheels, but a 36 spoke wheel is a whole lot stronger for very little extra weight. Buy your bike from a descent bike shop, who can check and tune the spoke tension if neccessary. Pick your shop before your bike brand.

Specialized Hardrock and Stumpjumper seem to be the most popular base-level for serious riding, but Giant, GT, Trek etc all make fine mid-range models.