The worst I have ever had was Lowenbrau in Africa. It is made locally under license, and tastes nothing like the stuff in America.
I've also had San Miguel in the Philippines. I found it horrible at first, but after a while you get used to it. The bottle reads, "The only beer that nourishes true Filipino friendships." Yep... everyone's your friend when you're blitzed. When I came home I found myself missing it.
Just went to their website and was amazed that they are one of the top 15 selling beers in the world.
I don't know what the difference was, but the rule amongst us was that if the bottle had a paper label, not to buy it - only the bottles with the painted labels. If any of you have had it, maybe you can enlighten me.