I'd have to say Warrior/Assassin (not actually my favorite because my favorite is actually a pure mage). Able to kill his enemies without conflict and yet able to kill outside of the comfort of shadows. Would probably be more powerful with crafting skills but this build is rather perk-heavy without crafting.
For Skyrim: I usually play Paladin/Necromancer/Vampire. So basically, the ultimate "Death Knight"
For Oblivion: Paladin(Heavy Armor, Healing Spells, Magical Defenses)/Alchemist (Potions that boost my skills and Attributes. NO POISONS!)/ Barbarian (Greatsword). Thats right, BloodBane is actually a nice guy (sometimes) XD
Morrowind: Have yet to truly decide what style to play as. Will DEFINITELY try being a good guy though... Then turn bad and try to kill everyone.
Mage + [Anything] = [Something]
Mage + Nothing = Best Class.
Mage + Warrior = Magic, to the FAAAAACE.
Mage + Rogue = Stabbing magic to the face.
Mage + Stealth = /whisper: magic to the faaaaace
Mage + Mage = GAWD-AH!
Mage + Bow = /twang: Magic! .... wait for it.... TO THE FACE!
Mage + Cheese = Sheogorath