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