Vanilla = Nordic Carved or Ancient Nord (dat side boob)
Too many awesome armor sets from mods to pick one.
Vanilla = Orcish
Modded = Nord Mail Hauberk (chainmail armor from Immersive Armors mod)
Like http://i4.minus.com/iblTotTMJa58zD.jpg, and http://i2.minus.com/ieUcrfCqFiX8M.jpg.
Vanilla = Ebony mail or Iron (Except helmet), ebony mail + ebony set for characters who would use medium armor/chainmail if it existed and Iron for my warriors. I also really like Steel plate.
Modded = not sure
I like Ebony for its knightly aesthetics and Nordic Carved for its Nordiness.
Steel Plate also looks quite sixy with Cabal's aMidianBorn textures.
I'm not a big fan of vanilla heavy armor, but if you ask my favorites? Emperor's Will (Imperial Knight for Immersive Armors) and the Drakul set.
Sadly, I'm a console player so I don't have access to any of the sweet mods I keep hearing about. My favorite heavy armor in vanilla Skyrim is definitely the Steel Plate armor (sans helmet). I rarely wear armor, but I always outfit my follower (usually Benor) with the steel plate and the dragon priest mask Otar and I think it looks pretty badass.
Modded = https://farm6.staticflickr.com/5561/14489962992_232cce273b_o.jpg (edited in the creation kit to make the saviours hide heavy armour instead of light)
Vanilla -> Nordic Carved
Mod -> Hedge Knight
I really like the Ancient Nord Armor Set minus the helm. Stalhirm (Also minus the helm) is another favorite of mine.