The best tarp in what respect? Just curious, I knew someone who has one they're thinking of shelving in favor of a tarptent.

I've looked a lot at bivies and tarps - some of the bivies are heavier than tarptents. There are a ton of options and some don't absolutely require a ground cloth. A bullet proof (heavy) bivy probably wouldn't really need one.

Gossamer Gear, while you're there, has polycro ground sheets. I've used a trash bag before. One of those cheap 2 oz plastic rain ponchos would be a dual use item - ponchos being what they are, even if it has a few holes it will still keep most of the water off you in the rain unless it's really pouring, which is when you should have real rain gear anyway... 2 mm painter's drop cloth is what I've made ground sheets for tents out of before, a couple bucks and you have a whole bunch of them for the trouble of cutting them out.

For snow you might look at mid tents instead - Oware has mids, Black Diamond too. Use with or without bugnet. Very good in snow, I hear.

I have two large tarps but I usually hang my hammock under them. I've pitched one on the ground in several occasions, however. It's caternary cut so shed the wind like a champ. If you anticipate being in a lot of precipitation you might think about where you're going to cook while it's raining or blustery and whether the shelter you choose has room for that - I've had pretty good luck under an open ended tarp with the closed end into the wind.
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