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 tend to rewatch comedies more because they tend to not be too story dependent so spoilers don't matter much.
Youjo Senki probably 4 times total. I think very few anime ever got the giga brain MC trope correctly ever since Code Geass. Also the fact Aoi Yuuki just owns the show.
Gekkan Shoujo Nozaki-kun 2x+ just great, terrible shame there is no 2nd season. Main girl VA is one of my favorites too.
Joshikousei no Mudazukai 3 or 4x because it is realy fun, literally all explicitly built on tropes and they all work.
Asobi Asobase 2x pretty much takes the essence of how Japanese comedy and rumps up to 11. The problem is that everything is so memorable I don't see the urge to watch more, still great for what it is.
Sayonara Zetsubou Sensei/Arakawa under the bridge at least 5-6x though not everything at once. So many good timeless gags, explicit shitting on Japanese culture is a bonus, also pretty much Hiroshi Kamiya fan paradise for me.
Steins;Gate 2-3x there is a lot of detail for you to pick up.
Fate/Zero 2x+ by far the best fate, realy the only fate anyone needs to watch imo. The Gilgamesh vs Rider final scene is one of the greatest in anime ever, also Kiritsugu vs Kirei. And this is from someone who realy doesn't give a shit about the whole Fate franchise. I still absolutely hate the bullshit ending nonsense so skip that.