Humped a M4, but if I had to grab one of my rifles for an extended time, I would probably grab my iron sighted 74 conversion to a 105. The 5.45 round is pleasant to shoot all day, you can still get 1080 round "spam cans" for less than $200 and the AK action is hard to beat for reliability.
For casual shooting, the m16 plaform is nice. Minor nit to some of the comments, I wouldn't really take the 7.62x51 out to more than 600m. The standard round goes trans sonic about there and the accuracy goes down. For out to about 1500m the .338 Lapua in a bolt gun is good (if you have the glass to see ANYTHING at that distance). Beyond that, call in some arty or an air strike.