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.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzukihttp://hikeandbackpack.com