The Beyond Skyrim team is currently in the process of building the various regions of Tamriel, so far they've released Beyond Skyrim: Bruma, which is basically just a demo of their significantly larger Cyrodiil project, despite just being a fraction of the intended project the mod on its own is still massive and of incredibly high quality and will feature its own full questlines. Their youtube page has some really incredible stuff on it, not the least of which is the six hours of music they just posted from their Iliac Bay project, the full scale of their work will probably end up surpassing the base game.
Best Skyrim mod though is probably Enderal, a full length game of its own set in its own world with its own lore and a main storyline that makes Skyrim's main quest look pathetic. It even managed to get its own Steam page that coincided with the release of a DLC for it that added more questlines.