Good question

For sweet vermouth, I alternate between Carpano Antica and Punt y Mes, depending on whether I want a sweeter, more traditional vermouth (the former) or something a little spicier and with more of a kick (the latter). Vya is also good, but expensive. M&R red is just a horrorshow, and I think Cinzano and Stock and the other usual suspects are all pretty mediocre. The experts really recommend the Carpano, especially in a Manhattan.
For dry (white) vermouth, I hear Dolin is unbeatable but expensive. Vya is also a top-shelf name, and I actually think that Noilly Prat, even though it's recently been reformulated, is more than acceptable. I don't think I could tell most of them apart in a blind taste test -- what's more important than anything, especially with dry vermouth, is that you don't allow it to oxidize. Only buy 375 mL bottles, and try to decant it immediately into even smaller containers, limiting its interaction with the outside air.
That's my two cents.