My favorite is http://en.wikipedia.org/wiki/AR_15 its nothing special or complicated, but thats what I like about it, its well known, reliable in both firepower and accuracy (I have fired one of these several times and never had an issue). Very similar to the standard issue M-16 which I also like, I just like the amount of attachments and customization you can put on these things. Scopes, grips, laser pointers/flashlights, etc.
Disclaimer: No arguing about politics, for some reason whenever war or something about war (like weapons) is discussed someone drags politics into it and messes everything up. Just talk about weapons, not whether war is right or wrong.
Thanks.