Necro is probably the most fun overall. Being able to FD, insta-invis and heal also makes the class much more forgiving of mistakes, leading to less frustration and the ability to take more risks. Necro also has a variety of ways to level which keeps things fresh: fear kite, root rot, charm are all viable XP options. And while duoing/grouping Necros shift into more of a support role, using different tactics and spells. Always a fun and interesting playstyle with a lot of room for variety and experimentation.
Enchanters would be a solid second place in my estimation. They'd definitely be in first place if having the most powerful character is your idea of fun, since they are unarguably the most powerful when well-played. But they're not easy to play well and tend to die a lot, which is not fun for most people.
I'd put Magicians in third place for fun. Most people will be at least somewhat turned off by their limitations, and some find the lack of variety in the Mage playstyle abysmally boring. But others find it a relaxing change of pace compared to more demanding classes. Really depends on the player.
Wizards are the least fun by far. Do not play a Wizard unless you are a hardcore raider, in which case you wouldn't even need to ask the question. In other words, if you have to ask whether or not to play a Wizard, you should not play a Wizard.
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