Can't tell the "best" marinade because there are so many really good ones. Just depends upon your mood.

Here is a really quick and very tasty standby...

Wishbone Italian Salad Dressing. It has to be Wishbone, and none else.

Soak a piece of meat in it for about a day or even two. Then through it on the grill. Don't overcook. Tasty.

For roasts and stews, throw in about a quarter to half cup of the stuff and cook. Adds a very nice zing to stews.

I usually avoid marinades for choice meats (like backstraps and tenderloins). I use dry rubs instead. But for roasts and stews, I have used the Wishbone treatment several times.