Two questions for you first.
What distances do you want to shoot...and how accurate do you hope to be. Meaning, do you want to hit a man sized target at 300 yards or are you wanting tight groups at 100?
How much are you willing to spend on a scope and a detachable mount?
I can offer you suggestions but need to know more first.
I have an EoTECH on my close quarters gun, a ACOG on one and a 4-16 Weaver on my long range AR.
EoTech doesn't need a mount since it's built in. Around $400.
4X ACOG and LaRue Detachable mount near $1000.
Weaver 4-16 and mount was about $500.
There are plenty of good choices and many bad ones. DON'T buy cheap chinese, your Colt will reject it.