I play around with several different tarps. From the Canadian tire special, to a few I've bought on camofire.com and some that came with my backpacking hammocks.
With all of the tarps I've used, I like the siltarp the best, as long as they have a bunch of tie-off points around the perimeter and usually along the midpoint and ridgeline.
There are a few Canadian manufacturers that have some pretty good lightweight versions. I do find that I've never complained about having too big of a tarp when in use, so for backpacking or sheep hunting I'd go with the largest that you feel ok with carrying.
Edit: Forgot to mention that one of my favorites has little pockets on each corner and at midpoints with about 10' of string already sowed into the tarp and it's pretty strong. I find it convenient to have these pre-built tiedown points.
If you don't mind a bit of crazy, a former AO member who was banned (rightfully so) had some neat tarp setups, but he was nuts (and militant) about his stove ideas.
http://www.outdoorsmenforum.ca/showthread.php?t=321267
http://www.outdoorsmenforum.ca/showthread.php?t=86150
http://www.outdoorsmenforum.ca/showthread.php?t=29857