first of all that question is very broad and very subjective. first you need to decide what exactly you expect from an AR, what you plan on doing with it, how much of a budget you have allowed for it's purchaase and whether you want one ready to shoot or what level at which you are willing to put together your own.
mine is a very basic, older Bushmaster 16" barreled, M4A2 Patrolman's Carbine, that serves my needs very well. i bought mine very lightly used and what i felt was a decent price. my next one will be built from the ground up, because i can build the one i want much cheaper with the features i want for than what i would be able purchase ready to go off the shelf. mnay will buy one off the shelf to become familiar with the platform and learn about their operation, assembly, cleaning and maintenance, before jumping in and building an AR. it's something i suggest for someone new to the AR platform and helps get them familiar with them and decide what the wnat or need and what they don't want or need in one. and that's one of the beauties of the AR platform, is that they are easily adaptable to just about anyone's needs or wants.