Yeah HP is a classic. I've been reading HP fanfics for over 10 years now but always come back to it after a while. I'm actually reading a Drarry fic right now, which was recommended to me on Reddit a while ago - really good and it has the extra points of being written by one of my favorite writers, which I actually fell in love with from another fandom.

It's really hard to say which fandoms I'm reading from, cause I constantly discover new ones - sometimes I read only a few fics from them, cause I only like a particular ship, sometimes I read only from that fandom for several months

The "bigger ones" I can remember right now are Harry Potter (I have some ships that I love but I've tried numerous other ships and combinations. The HP fandom is just so versatile!), Game of Thrones (my favorite ship is Jaime/Brienne but I've read many other ships from that fandom), Succession (I mostly only read Gerry/Roman here), ATLA (again, I have a few ships I prefer reading), MCU (very versatile, I don't really have a favorite ship here), The Expanse, the Jane Austen fandom etc.

It's difficult to say where my "bigger" fandoms end and my "smaller" ones begin. I've read a lot. I always read from one fandom or another, sometimes from several ones at a time. I'm mostly a "ship reader" so I usually go for fics about particular ships/couples if I find the couple intriguing, regardless of the fandom. The "bigger ones" I listed above I always go back to every once and a while, but sometimes I remember or rediscover smaller fandoms that I feel like revisiting.

That being said, my "shipper mind" always works its way in every piece of media I see or read, so a new ship is always waiting to be discovered by me everyday :)))