Any AR15 you buy will be just as upgradeable as any other AR15, so it does not really matter what AR15 you buy to begin with.
In my opinion the 2 most important things to first consider when buying a pre-built AR are going to be to get barrel length/profile you want and sticking with a flat-top upper receiver so that you can mount your optics.
Of the brands available, some would consider Colt and LMT the best, but there are lots of other good brands such as Smith and Wesson, Bushmaster, CMMG, Stag, Olympic, etc. You really cant go wrong with any of them and they will all work just as good as the Colts and LMT's.
Some AR15's already come "pre-modified" with things like Railed Handguards, Buttstocks and Pistol Grips. IMO I would not purchase one of these because chances are you will like a different buttstock, pistol grip, BUIS, etc better than the one you purchase comes with anyways.
I would just stick with just a basic AR15 with Plastic Handguards, stock Pistol grip and Stock Buttstock and then shop around online for the parts you really like and then add them later, none of these addon parts are very hard to install exept for some brands of handguards.