Re: What AR .308 is the best to buy...................
First off, what is your budget? We can all take stabs at "guessing" what may or may not be the best choice for you, but knowing how much you want to spend on your rifle, mounts/rings, optics, bipod, etc., etc. will go a long way toward pinpointing the best options out there.
Second, what are your expectations with a 308 gas gun in terms of accuracy? Are you talking 1.5MOA, 1MOA, sub-MOA?
I will tell you that if you are "wanting groups like [your] bolt guns" that it is possible, BUT you need to throw the "affordable" idea out the window and concentrate on quality components for a custom build. It is tough to get a reliable (100%), highly accurate rifle out of the box from the factory unless you definition of "accurate" is MOA or so.
As long as your skill set is where it should be, and you are running decent ammo, a custom gas gun can put you into the sub-MOA accuracy range without much difficulty. While there are factory rifles capable of the same, there are diminishing returns to spending the kind of money on a factory rifle when you can go custom for not much more money.
If you are going factory, wanting sub-MOA accuracy, reliability, etc., then the POF P-308 20" SPR is an excellent option, but not a cheap one.
Again, let us know what you want to spend and folks here may be able to get you closer to a specific rifle/platform to consider.