Unlike this guy I have precisely 168 separate mods running at once - I actually counted them
And I still haven't installed an entire suite of mods designed yet designed for Lakeside Manor, because I've not yet built a home on this latest character. And that's not counting all the things I have installed to make the game look pretty, that are not actually mods....
Modding is as addictive as hell, and there's always that temptation to install just ... one more... because heck... the default fur on vanilla bears is so dull and, there's no way I could ever play without the ability to hold a fish in my left hand and, there's so many new mods coming out all the time and ....... ooH shiny!
Now, on a more serious note. If there is one single mod that I absolutely could not live without - that would literally break my game if if were ever removed, it's this one:
http://www.nexusmods.com/skyrim/mods/18021/?
Blacksmith's Storage Chests
It's a simple thing, but it makes so much difference. Basically, you purchase the chest in a Hold, such as Whiterun, and with a single mouse click you are able to safely store all your blacksmithing materials - ore, ingots, hides, leather, dragon parts etc. The chest automatically sorts through your inventory, finds all the above and instantly stores them for you. Opening the chest gives you the option to retrieve all or some of your items. This makes life in Skyrim so much better that it's hard to quantify. What's more the chest can be toggled so that your materials can be accessed from any chest in the world you have purchased, including the one in your Hearthfire home(s).
If you never use Blacksmithing it's likely not much use, but if you do, you are seriously going to need this.