I don't know about "best", but I've heard members here using "blue grid" (from Coastal Business) and dark transfer paper from tshirtpaper.com. They also have some opaque paper at personalizedsupplies.com
You may have to try out a few to see which you prefer to work with.
The dark paper from personalizedsupplies.com is horrible. Don't use it. I like tshirtpaper.com's paper. It has a blue grid on the back of it as well, so it very well may be the same paper that coastal sells.
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