Isaac, as Mackay said,you need to define your needs. I myself could not get by with just one AR. I have several ranging from the 16" RRA M-4 style to custom built 16"and 24" flat tops for the bench and LR varmit hunting.
If you were to go with just one all around gun I would choose the Colt 6920 or the newer 6940 and call it good.


[Linked Image]
"Until one has loved an animal, part of their soul remains unawakened."