SkyRE is fairly casual in the mechanics it buffed from vanilla, but the combat itself is the hardest of the lot, as enemies are extremely intelligent now with the latest update. Not only do they block pretty much everything you throw at them, but groups will flank you, and even the biggest, burliest warrior has flanking bonuses from the sneak tree, it's immense as a challenge whilst being accessible on the mechanics side to a casual player.
I've never heard of Requiem, but ACE was the first ever overhaul and has had a lot of work done on it, so it's the safest bet if you're gonna slap an overhaul onto a pre-modded game. Non of the overhauls will work with existing saves, though.
I personally say SkyRE, 100%. It adds everything you could want, it adds hundreds of weapons and has complete compatibility with immersive armor. Also, get Weapon Variation Expansion, that's another incredibly good mod, it basically takes all the vanilla weapons and randomizes their looks, so you could end up with an iron War Axe that actually has a blade on each side, instead of it being just a Hatchet, and you can find the Zweihander looking Steel Greatsword but in Longsword version, which looks awesome.
W.V.E Mod
This 'WAR' Mace look familiar?
