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
I will rewatch The Melancholy of Haruhi Suzamiya and Kill la Kill at least once a year at this point.
Every few years I'll watch through Dragon Ball and Dragon Ball Kai (used to watch Z and skip fillers before Kai was finished)
Also somewhat recently during the first wave of Covid lockdowns, once a month for 3 months in a row I rewatched the first 3 seasons of My Hero Academia. Not really sure why I did that
K-On!, Toradora! and Little Witch Academia are all comfort shows I'll throw on somewhat regularly when I'm in the mood as well as the first 30 or so episodes of Ranma ½ and some of the OVA's.
I love the Haruhi series and movie so much. Once I start it again, I end up binging it pretty quickly. Whenever I listen to the OST I can picture so many scenes vividly lol
Toradora has been on my list forever. I really need to watch it