It's definitely a toss up for me. I think at this point I've seen The Melancholy of Haruhi Suzumiya more than any other series. It's pretty much my go to series whenever I'm down. It has it's issues, but I really like the characters
I've also watched both Boogiepop series and Bloom Into You many times as well
edit - adding D-Frag as well for feel good series I watch almost once a year
Macross Plus, for the music, the great dub (Bryan Cranston as the protagonist), and the fact that it was one of the first anime I ever watched. Oh, and I fuckin love transforming robots.
Ninja Scroll, for literally being the first anime I ever watched. If anyone here remembers the premiere of the Teletoon and Space networks in Canada, they used to show anime on Friday/Saturday nights, and their selections, though limited, were surprisingly good. I caught NS while channel-surfing and literally went from zero to filthy, unapologetic otaku in 5 minutes.