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
Gundam Wing for sure - having the series on a thumb drive means that whenever I don't have an internet connection for a period of time I inevitably watch it again.
That's one of the few that I've watched a couple times, it's weird because I've watched a bunch of other Gundam series and I get what people mean when they say that Wing is one of the lesser entries in the franchise, but it's the only one I go back to regularly whether that's listening to the OST or rewatching it.
Definitely - while Wing has purchase in my brain because of Toonami nostalgia, it is an admittedly weaker entry. But even knowing that it's just super re-watchable for me.