RE: Best Varmint Rifle and Caliber?
.22-250. As far as the gun, you'll be hard pressed to set up a decent centerfire varmint rig for under $800. Is $800 your total budget, or just for the rifle? Remember, a good varmint rifle needs good glass on top. I'm a fan of the Nikon Monarch 6.5-20x44 (don't know if they still make that exact scope anymore, though), but one will set you back about $600-650 just for the scope unless you can find a really good deal.
If I were shopping for a varmint rig I'd either be looking at the Coopers, but they're going to be a bit more than $800.
Mike