By essential I mean mods such as SkyUI(unless there's something better now?) and unofficial patches. Things you wouldn't want to play without, and aren't a matter of taste as much as a near unquestionable improvement. And of course .ini tweaks, I remember some basics like setting mouseacceleration to 0, but IIRC there are several visual and performance improving ones as well as ones that make gameplay better like the archery tweaks.

And visuals, well, the main reason I'm starting Skyrim up again is 1. DLC I haven't played, and 2. my video card died and I replaced it with an nvidia GTX 670 which I imagine should allow for quite a few demanding visual enhancements(ENBs with SSAO - or would it be better to enable SSAO w/NVIDIA control panel?) relative to my old radeon 6950. So I'm hoping for the works.

Frostfall. It would be hard to express how much better the game it with this in it. I honestly don't think I'd play without it. Climates Of Tamriel is a fantastic weather mod. Immersive Armors and Weapons, and everything aMidianBorn. There is also a mod author named InsanitySorrow and you pretty much want to use anything she touches. Also, Realistic Needs And Diseases is a fine addition to the game. Skyrim Floral Overhaul is wonderful as welI. I would check the top 100 mods on the Nexus, read the descriptions, and decided whether they would be essential to you.