My experience with Colts is much more extensive than the other brands. About 95% of my exposure to AR-15's is from the law enforcement side of it. As a police armorer, I personally see a very high degree of quality control coming out of the Colt factory. It's very rare to see Colt lemon, however I've been around long enough to know that they almost certainly exist.
I own several personally owned AR15's, and they are all Colt. Call it brand loyalty, but I go with what works, and Colts do.
There are a bunch of people who will laud Bushmaster, Rock River, Olympic, DPMS, etc. as being superior to Colt in one fashion or the other. The bottom line really is that they are all pretty darn good. I do however see Colt owners doing a whole lot better getting their money back out of Colts when they decide to sell or trade.