I prefer the ACOG. but they are pricey.
Leupold, Nikon, Nightforce all make good units.
Personally, I would avoid anything made by Tasco, Bushnell, Simmons or anything else in their category. Good glass isn't cheap, but it's well worth the time and effort of saving up your money for a few months.
And for a 5.56X45, the 1.5-4X20 is near ideal. 2-7 and 1.5-6 are good. But you don't need 4-12 or even 3-9 on a .223.
For .308, 2-7 and even 3-9 are okay, but again, anything over 9x, IMHO isn't needed.