The longest lasting fun I've had was with my sneak mage, who utilized Destruction, Conjuration, Restoration, Alteration, Illusion, Enchanting, and Sneak. what made it so enjoyable was having many methods of confronting any combat situation.
I could do some old fashioned destruction spamming if I wanted some mindless mayhem, or I could cast calm on a large group, followed by invisibility and sneaking out quietly. If I wanted to dominate the situation, I could cast calm, then mass paralysis, then summon two Dremora lords and let them beat up my fallen foes.
It was a pretty flexible build, and it took a lot of perks, which is good because part of the fun in Skyrim is the buildup.