ProTip: If you re-map the "F" key in Skyrim's key bindings, the new key to assign a favorite is "A." This isn't explained or listed anywhere in game; we found it by accident.
What Makes a Mod 'Must-Have?
There are literally thousands of Skyrim mods available, ranging from small adjustments to NPC dialog or tone of voice, to expansive projects that alter core gameplay and introduce new spells and visual effects. We've prioritized mods that showcase the technical capabilities of the PC, provide a more balanced game experience, fix bugs, and improve Skyrim's UI. In some cases, particularly when it comes to customized post-processing filters, there is no definable "best" -- just the question of what players prefer.
The mods in each section are listed by the scope of their changes rather than being ordered from best to worst. A mod that changes the behavior of one NPC would be farther down the page than a mod that lets you play as Pinkie Pie, from My Little Pony. We've also noted which mods change gameplay and which do not.
How To Get Started:
There are two basic ways to get started with Skyrim mods, and two main websites devoted to user-created content. If you bought the game through Steam, you can use the Steam Workshop. Login, Click on "Community," then "Workshop," and you'll see two games listed: Team Fortress 2 and Skyrim. Installing mods is as simple as clicking on "Subscribe" to a product that you want to download. Steam offers mods both individually and in collections.
The other method is to use the Skyrim Nexus website and the Nexus Mod Manager it offers. The Mod Manager offers players more control over how and when mods are loaded, as well as an organization function that automatically arranges mods in the order necessary for them to load properly. This last option is extremely useful for play testing and bug-fixing and it's what we primarily used for our own evaluation.
Skyrim Nexus has more mods available than Steam's Workshop as of this writing; a number of the projects we recommend aren't yet available on Steam. If you're interested in trying our recommendations via Steam, we've collected all of the available mods into a single package; available here. For the moment, Skyrim Nexus is a better option, though Steam's one-stop-updating is rather handy.