1) If you CAN hit 12" pie plates at 300 yards using iron sights, you don't need a scope.
2) If you CAN'T hit 12" pie plates at 300 yards using iron sights, you need more practice.
Iron sights work. Don't dismiss them.
If you just have to have a scope, you need to determine what you're going to use it for. At what ranges and at what level of urgency.
Long range paper punching and varmint shooting, you can get away with using a high power (10X+) scope with a skinny reticle (crosshair); look at Leupold Vari-X series ($300-$700 as long as you stay under 14X).
If you want combat versatility, try Trijicon ACOG 4X32 ($800).
If you want close quarters battle (CQB) speed, try Trijicon Reflex II ($300).
Some people prefer red dot sights such as Aimpoint Comp M ($300-500).
The proper mounting hardware for any of these should set you back at least a couple bills.
If you have a fixed carry handle upper, forget all desires to mount scopes on top of the carry handle. Line of sight will be way too high for any kind of decent point of aim/point of impact correlation.
I also noticed that you used the words "best" and "affordable" in the same sentence. This combination does not exist in the world of scopes. Anyone telling you otherwise is either trying to sell you crap or doesn't know what they're talking about.
Finally, what ever you do please DO NOT put a $40 scope on top of your AR. You might as well buy a Honda Civic and put cheesy graphics, gaudy body work and a 5 foot high spoiler.