I actually like all the classes and look forward to playing each one, but if I could only play one forever after, it would be the magic-user. The sheer variety of tricks they can bring to the table is what I like the best. You can burn groups of the enemy with fireballs. You can cross that chasm with a fly, a teleport, dimension door or a wall crawl spell. You get a friend with your familiar. You get to change the world through the spells; earthquake and create magic item.
Magic-users are the one class that is truly limitless. Magic-users have the widest range of possibilities and that is what draws me to them.