There are quite a few that I like, but if I were forced to pick just one, it would be a Guinness that's been PROPERLY POURED from the tap. Seriously, if you drink Guinness often enough there is actually a noticeable difference between one that has been poured properly and one that has not. In fact, if you go to the Guinness Storehouse in Dublin there is a point in the tour where they will teach you to pour one properly, and give you a certificate.
To the people asking about mead, as with a lot of beverages it will depend on the brand. The one I tried was fairly decent.