What would you most like to change in the game?
People will suggest the mods they like the most, but the beauty of mods is that you can personalise the game to suit your own tastes.
Do you want different visuals ("better" graphics)? Do you want to change game mechanics in any way or are you happy with the general way it plays? Do you want a more challenging game or one that makes certain things easier? More roleplaying? More weapons/armour? New quests?
The only ones I think most people would recommend are the Unofficial Patches by Arthmoor and SkyUI for a better user interface if playing with a keyboard and mouse.