pretty much the most op build is the stealth archer. Just level up smithing, and enchanting to make your equipment overpowered and you can kill dragons in a few arrows. (level up enchanting then put as much grand soul gems as you can into extra archery dmg: ring, necklace, helmet, gloves) and also level up sneaking.
Being a melee type character is quite boring and being a mage is quite annoying and difficult (I love playing mages in games, but its just a pain in skyrim and the majority of spells are just useless, not to mention fighting other mages that use infinite mana and just spam wards that block magic also ruins it.
All in all my favorite build for skyrim is the dark elf stealth archer/assassin for how effective it is (not to mention the best guild is the thieves guild, the other guilds are quite dull especially the college of winterhold and companions).