Having an AR-15 (essentially a semi-auto version of the M-16) in your survival gun inventory is a good choice regardless of whether you choose a Colt AR-15 or some other brand.
That being said, actually choosing the best AR-15 can be a daunting task if you aren’t familiar with all the options or the vocabulary that has built up around this rifle.
For example, you may find sellers that claim the weapon is “Milspec”. Supposedly, this means that the gun was suitable for military use. According to Colt Firearms (the owner and trademark holder of the AR-15) there is no such thing as a legal Milspec AR-15.