I'm thinking about getting an overhaul mod this time around and was wondering which one people think is the best. Whichever one has compatibility with other mods such as immersive armor or interesting npcs is also nice.

I used ACE for a while, and it was pretty good. It's a lower footprint kind of overhaul, modular, doesn't have a lot of compatibility issues.

After 400+ hours of playing Skyrim, I wanted something that changes and adds more, so these days (and for quite a while now) I use SkyRe. It's also modular, the main core of it being the perk changes, which I think are fairly well balanced and interesting. There are also AI modules, encounter zone changes (which I don't use), Standing Stone changes, Race ability changes, etc. The issue with any overhaul is that, while you might like and agree with some of the changes, you might not like others. For me, SkyRe makes good overall design decisions, and that's why I use it. A lot of people will tell you that compatibility is an issue, but I use lots of mods (including the two you mention), and haven't had any issues. For most things, it just works, with no extra effort from you. For a few others, like Apocalypse Spells, there are patches.

That's all I've used personally. From what I understand from reading about it, and reading discussions about it, Requiem is favored by the "hardcoe" players who value realism. It changes too much for me, and I don't like all of the changes. SPERG is a perk overhaul only, I believe. It might be great, but I'm happy with SkyRe. ASIS isn't an overhaul. It allows NPCs access to the same spells and equipment that players have access to. It can be used alongside any of these other mods, IIRC.