SKSE (duh), Frostfall, RND, SkyUI, iHUD, Wet and Cold, Immersive Armors, Dual Sheath Redux, Duel Combat Realism v5, Winter is coming - Cloaks, the Unofficial Patches. These are the most important mods for me. I could also mention the recently taken down Climate Overhaul - Winter mod. Paired with Frostfall it makes Skyrim an even harsher place, and since it completely changes the areas that weren't covered in snow before, it makes Skyrim feel fresh. Oh, and due to the amount of times I've been killmoved at high difficulties Violens has became an essential mod for me (you can finally toggle Player killmove immunity).
As for graphical mods I love Kinematic Enb and ELFX combined (interior module only), and also most of the popular high-res retexture mods (Skyrim 2K, Amidianborn, SMIM etc.)
Calipers, they're a must.
This is the compatibility list. I was looking at it but figured I'd link for anybody watching this conversation.
Unfortunately I have mod clashes if I were to try Requiem.
You should take a look at the Patch Central then, if you really want to try the mod that is.
But we'll have to wait till Azirok update the patches. Requiem 1.8.1 has been released a few days ago and there is a shit load of work to be done with those patches.
If you want Skyrim to become like Morrowind or Dark Souls, you should really give it a try.