did you try searching in this forum? that would have been fruitful, since this exact question comes up fairly often.
cantilevers are the way to go for flat-top ARs. there's a good comparison of many brands here
; the burris PEPR is touted as the best under-$100 choice. above that, the options i've seen tend to be pretty nice -- i use an ADM that i like a lot.
“Keep it up, America, keep telling your youth that mud and danger are fit only for intellectual pigs. Keep on saying that only the stupid are fit to sacrifice, that America must be defended by the low-brow and enjoyed by the high-brow. Keep vaunting head over heart, and soon the head will arrive at the complete folly of any kind of fight and meekly surrender the treasure to the first bandit with enough heart to demand it.” (Robert Leckie)