It's gotta be The 100 Girlfriends Who Really Really Really Really REALLY Love You. Starting out, you wish you were Rentarou. By the time you've finished the first couple season you'll just wish you were dating him instead.

Reverse? Ouran is a classic for a reason. Btw, for those who don't know the Ouran anime doesn't even cover half of the manga so I'd definitely recommend giving it a read as well.

Monogatari series, anyone?

Watanare, Dokidoki Precure, and Hamefura are probably my favorites....but like. I found myself questioning whether half of the works listed actually fit the diagnostic criteria of 'harem' or 'reverse harem' when I looked them up on anilist. Because in a lot of cases it's just that a lot of characters like the protag, but the protag's feelings back aren't clear. And in many cases it isn't a polyamorous relationship or anything, it's just that a lot of characters like the protag, but then there's the implication that the protagonist needs to choose and that's like. antithetical to the concept? so there's not that many works tagged 'harem' that end up being a true harem in that sense. So I struggled to respond to this since a lot of the time, it's just a lot of characters competing for a monogamous relationship, which in the end, doesn't actually feel like a 'harem' even if you could describe it as such during the process of 'elimination' so to speak???? maybe I'm just thinking about this too much though. I don't know.

Serious answer: Katalina.

Joke answer: Doki Doki Precure.

This is an unconventional one, but clearly fits

Sayonara, Zetsubou-sensei (SZS).

I just love the cast. The wordplay is amazing and the fan translations that I read from years ago have so much passion explaining complex Japanese word play and cultural references and SHAFT managed to adapt 3 seasons and 3 OVAs for it before they exploded with Monogatari series and PMMM to the point that I resent them for not doing anymore SZS lol

I gotta stick with the classic Tenchi. But I'm sure this surprises nobody.

Yes it's campy. Don't care lol.

Reverse? Gotta be Ouran High School Host Club. Regular? Sword Art Online because real ones know it isn’t one at all. Kirito is just a ladies man. There’s his fiancé, her best friend, his sister, his friend who he sees as a sister and an equal, and his AI daughter. Then there’s his best friend, and his boyfriend.

does Outlaw Star count? lol idk any others

The world god only Knows was the first one I watched and does have some good moments.