Optimator is a phenomenal beer, and one of my personal favorites. It's a doppelbock, so not really directly comparable to Guinness (an Irish dry stout) except in that it's very dark. For that matter, Spaten's Munich Dark is quite good as well - a bit less sweet, though, and I think a bit lower alcohol content (Optimator is pretty strong).
Another good doppelbock you might enjoy if you like Optimator is Paulaner Salvator.