depends what you need...if you're a distance backpacker and need an ultralight one then the integral designs siltarp2 (10'x10', 13 oz) is the ticket...it's on sale for $107 right now. if weight isn't as much a concern but you're still carrying everything on your back then i recommend the bushcraftusa coyote brown 10'x10' ($60, 1.5 lbs). if you're stealth camping or otherwise sleep close to the fire then the bushcraftusa multicam 10'x10' ($115, almost 3 lbs!). if you're car-camping then a $15 one from the hardware store is just fine - only problem is it's a bit "crinkly" so tie it down really tight to reduce the noise when it's windy.
i've used them all in sun/rain/snow and they're all good to go except the siltarp2 which i just got a few days ago...i'm not a weight weenie, i bought the siltarp2 to cut weight so i can bring more knives instead