Depends on what you want to do.
If you just want to plink..........pretty much any AR will do. (Colt if you want the best)
If you want to get into Service Rifle, any lower with a good trigger, and a White Oak Upper. Good to 1000 yards. (Just an example, my service rifle trigger cost a bit over twice what I paid for the lower). $800 will get you a super White Oak upper.
If you want to do some 3-Gun style shooting then one of the flat top shorties or Civilian style M4 with some sort of red dot sights.
All will with in your $2500 budget, So take the rest, buy some reloading gear and components and load good bullets.
No AR or any other rifle is worth a hoot if you shoot crap ammo.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071