There are a lot of good options out there right now - as far as an M4 / AR-15 varriant, there are a whole bunch of manufacturers out there who produce a high-quality product. I personally don't care for Colt too much, given some of their marketing / manufacturing decisions lately, but it's a good rifle. Others I'd look into are Rock River, DPMS (or DMPS, can't remember), Panther, Bushmaster, along with a slew of others. For other non-M4 type weapons take a look at the SiG 556. If you're looking for a gun in a 7.62 - class round, I'd suggest a PRT-91. It's a H&K G3 clone that many people like more than the actuall G3. Bunch of accessories out there for it, and most G3 stuff will work with it. Price-wise I think new ones are in the 1200 range, which is significantly cheaper than an M1A / M14. That and mags are frikin cheap.
Hope that helps.