What type of shooting will be involved and what range? How much are you willing to spend? You can go with 1 of the many red dots out there ranging form 80$ for a tasco to over 200 for a leupold. eotech and atn also have the holo style sights. If you want a scope barska makes a decent tac scope in 1.5-4x for around 60$ you want to spend more burris nikon luepold atn trijicon make top quialty tac scopes in various power ranges. The burris scout scope is a nice compact 2.75x that would do perfectly.
If you're looking for a Red Dot, I'm pretty happy with my Tasco ProPoint PDP2. I got it for around $80 and it works great. It has a nice crisp 4 MOA dot and is nice and simple to use. In hind sight I probably would have gone with something a little larger. The PDP2 is 30mm, which works, but I think a 42-50mm would be a better idea on a 'tactical rifle.'