Is there any good substitute for sherry in a pot roast recipe?

Marsala is good, but NOT the green bottle California stuff. That's terrible. For a pot roast you are trying, through slow cooking, to develop some complex flavors so all the ingredients in combination, matter. Different sweetish wines are going to work with different herbs.

I'd lean more towards using a marsala or a sweet vermouth ...

My personal favorite is (reduced) red wine however brandy and gold rum are excellent too.

Ruby Port would be delicious

Perhaps red wine or beef broth.