I have way too many to say and I'm unsure how many of them would count as must-haves for most people. However, for me, I'd say a lot of my must-have mods come down to magic mods because a ton of my characters end up using different sorts, so I like variety, such as:
Honestly, I keep forgetting to disable some of these for a couple of my playthroughs so a ton of spells scattered all over just became the norm, even if I don't use most of them or if I'm playing a character that hardly even uses magic.
Ooh, a lot of these look fun! I haven't tried too many of these, but I can personally vouch for Apocalypse. That mod ALONE adds so much variety to the world of Skyrim's magic. Some of those spells remind me of something you'd find in an ARPG like Diablo or Fate.
It's hard to choose a favorite out of all of them, they've also been in my games for so long that sometimes I can't remember what spell comes from which mod unless it's super obvious.
Forgotten Magic adds rings as well that buff up some of the spells which I thought was a nice addition. One of these mods adds a table where you can make spells (buying/finding two or more spell books to create a new spell) but I don't remember which one does that, also pretty sick though.
Oh wow! I have to ask, do you get unusally frequent crashes with those merged spells? I've been trying to find a stable mod that allows some form of spell creation like the old games for a while and haven't had much luck.
I don't think I've ever had problems with how many magic mods I have, I hardly get crashes as is and my last playthrough was sitting at around 450+ active mods.
The spell making isn't like Morrowind's or Oblivion's spell making, but I enjoy it as a stand-in. The spell making tables aren't everywhere but there's one in Winterhold & the Abandoned Shack. It's bugging me that I can't remember which mod the spell making table is from, haha.