The two bigger brands are Sagres and Super Bock. Personally, other than Czech beer and Guiness I can't say I've ever found beer that I liked as much as the two portuguese brands. Oh, and Lapin Kulta was also pretty good. As for bad beers, American, Spanish and Swedish are the worst. Particularly Spendrups.
Funny sidenote: In Portugal, a lot of people swear by either Sagres or Super Bock, and won't touch the brand they don't like. Anyway, my mom once did a market survey on beer and found out that nearly everyone who was dedicated to one of the beer brands, actually couldn't distinguish one brand from the other when doing a blind test.