I have a vortex optics LPVO on my Ruger .556. So far its one of my favorite scopes, just because its 1-6x power, and adjustable on the fly. I would be careful with going anything over 1-6x, because the picture can start to get blurry the farther you zoom in. I've got the Vortex Strike Eagle 1-6x, which has been working great for me shooting at around 400-500 yards. If you want something a little heftier, the Arken scopes are pretty good too. A good mount for the scope is just as important, so you can maintain your zero.
I once made the mistake of getting a sig red dot and a 6x magnifier; it was terrible. It was super blurry, terrible eye relief, but it did look cool. It was nice to have a little flip up magnifier, but its a route I will never go again. Having a clear, reliable sight picture is one of the most important things you could have, and I think having a scope with clear glass would be the best.
Of course my previous options listed are for outdoor shooting mainly, but you can never go wrong with a Holosun reflex sight or just a regular red dot with a large field of view. It all kind of depends what type of shooting you plan on doing. If you're thing of long range, maybe getting an 8-24x scope might be the way to go. Optics Planet is a great website to get rifle scopes for a pretty good price.