You'd better choose the class first
Choose the style of play you prefer - melee, archery, spellcasting whatever. According to that, choose the class. There is no best class/kit in the game, nearly all of them are playable, balanced, and interesting to play. In my opinion, it would be cool to RP an evil, Irenicus-like mage. You know, ignorance for those pathetic peasants... My next solo char will be an evil sorcerer