It really depends on what you want to do with it. For a general use carbine, I've mostly used various Aimpoints. PRO and T2. Also a fan of the Trijicon MRO. It's easy to hit torso sized targets out to a few hundred meters quickly with an unmagnified red dot. There are a lot of very nice LPVO out now ranging from 1-6 to 1-10. I like Nightforce and the Vortex Razor line. A buddy has a lower priced Steiner that he really likes and I do as well too.
You really need to answer a few questions first. Decide what you want to use it for and your budget.