My two favorites are the Aimpoint PRO and H-1. If you're patient and search your ass off, good deals can be found. Check the Equipment Exchange sections of the many firearm forums. Even Craigslist and Backpage will have the occasional find, especially if you're willing to expand your search area. Get the best mount you can find.
I am also VERY satisfied with my Primary Arms red dots. At $100 or so they are a great value. A better optic than all others under $250 IMHO. Google the reviews, well thought of by several SMEs.