These are PC mods(I have to assume you didn't know that as most console gamers don't even know about mods, let alone that hey are PC only).
ACE- full on game overhaul. Combat, skills, speech, pretty much the whole game gets reshaped and made better. Skills make sense.
Duel: Combat Realism- makes combat much harder, stagger chances, decreased stamina regeneration, a lot more changes but all to combat.
ASIS- makes AI enemies use perks, and spells from mods, gives them poisons to use, and makes them smarter.
All three increase the enemy AI and make them use their numbers more. For instance, three bandits will now circle you like wolves instead of waiting to gihht you 1v1 in a single filed line.
Also I use Skyrim Monster Mod which increases the difficulty of monster fights (trolls are no fucking joke).
Sands of Time gives a chance while waiting, sleeping, fast traveling and walking to get an encounter.
Occupy Skyrim and Radiance both add in tons of new random encounters too.
I was running Skyrim Monster Mod - More Spawns, which increased any normal spawn (not a boss mob) to be tripled. I was enjoying this until I added in all of my extra encounters mods. Also usea folder mod called Ultimate Follower Overhaul, it just gives you some more freedom with you companions and you can bring more along with you. Also has horses for your companions. I only really bring one person with me but the extra control and options along with the horses are just perfect Imo.
Don't forget Deadly Dragons for those Dragon fights. But I rarely ever start the first quest and don't have it installed. I melee fighting a dragon with Deadly Dragons is very hard lol.