The original. I know, it's not a very original or interesting answer, but I tend to fall for the first in a series simply by the nature of it *being* the first and therefore becoming a nice memory in my head of when I discovered an awesome new movie (series)!
But the original Friday the 13th is a must-watch summer classic for me. Yep, summer, not Halloweentime. The camp scenario is so fun. And of course on any actual Friday the 13th, even if the connection is only in name.