Unlike traditional rifles, the AR series has a straight line stock. Which helps with recoil attenuation (no pivot point for recoil, it comes straight back). However, this means that your eye is now 2.5" above the bore rather than having drop to the stock that puts your eye right above the bore on traditional stocks. That is why the AR has such tall sights.
You will need to have the centerline of the optic somewhere around 2.5 inches above the bore. The barrel centerline is roughly 1" below the rail.
So a base that puts the center of your optic 1.5" above the rails will get you pretty close. (If you are using a magnified optic)
A RDS you will need to look at other considerations. (What cowitness if any you are looking for with your iron sights)
For "tactical purposes" Larue is the way to go. (ADM or Bobro as well but I like Larue the best)
For a hunting optic, I like Armalite 1 pc base.
The PEPR gets pretty good reviews at Optics Talk, but I have never used it.
I actually have a UTG that I used to raise the height of my Primary Arms M4 on my M&P-22 to get a lower 1/3 cowitness to match my "real" AR. (I bought 3 of them until I found the height I needed)
It is serviceable, but they are using a very low grade of screws with the attachment bar. I don't have really good feelings about the long term durability of the screws. They aren't properly heat treated and they will stretch over time. Possibly loosening up right before you are getting ready to make that really important shot. It doesn't bother me on a .22, but I would be hesitant to use it on anything with any significant recoil/weight optic on it.
The other disadvantage of the UTG is that you have to buy a separate set of rings and the base. More failure points to loosen up.
The one piece bases listed above have integrated rings with the base. So you eliminate one point of failure.
Hope this helps.
