If you want combat that makes opponents realistically react when getting hit and that emphasizes player skill by adding new moves (timed blocking and warding, hits in the back do extra damage...) then try Deadly Combat. (By Borgut). If you don't want fast-paced fights do not try it.
If you want a combat that is more based on character's skill and that feels like a natural expansion of Skyrim's original combat system, then try Duel - Combat Realism (by LogRaam). Duel makes opponents smarter by using the core mechanics of Skyrim's original combat more effectively. It also adds RPG depth to weapons by giving them higher chances to stagger opponents depending on how much they weight.
If you want to turn Skyrim's combat into the most realistic experience possible (i.e. a combat simulator) then try Duke Patrick's Archery and Heavy Weapons mod. Duke Patrick has decades of experience using real medieval weapons and armor and he has used his vast knowledge to turn Skyrim's combat into something believable. The mod adds tons of features, one of the most celebrated ones being locational damage (hitting the head does more damage than hitting an arm). A word of advise: Duke Patrick's mod doesn't go easy on the player. It brings hard realism and some players may find it more annoying/frustrating than fun. Others will completely love it though.
is Duke patrick's mod fast paced like Deadly combat? i use deadly combat but have been really tempted to use duke's mod because of it features, but i really can't stand a combat like the vanilla one where i hit someone with a greatsword 20 times, the enemy barelly has any armor and he still fight and taunt me like i'm a little girl swinging a wodden stick '-', that's why i love deadly combat, 2/3 hit's having a few perks, even at high levels almost any men die, even myself *-* (a real pain to fight against wizards but i love the challenge >D)