I'm a long-time fan of Escaflowne. It was the first anime I really appreciated in all its undubbed glory.
If you're looking for which ones are ultimately the most popular, I'd say that older anime--like Evangelion and Dragon Ball (any form)--would be your best bet, though newcomers (by most standards) like Bleach and Naruto probably aren't all that far behind. And of course, there's always the Pokemon franchise to consider, too...
Bookmarks