I'd have to say Bushmaster. Colts are nice, but to me they have seen their better days during the "pre-ban" years. Colt is also NOT a pro-2nd Amt. rights company. Olympic is terrible. DPMS is a company I've heard good things about. They produce an all-stainless steel AR lower.
Based on my experience, I'd have to go with Bushmaster. Good quality and customer service.