I'm partial to wizard, but it's the only class I've played past level 40, so I might be a little biased. It also has nothing to do with the fact I'm trying to sell my high MP gear for around 400 mil...
The reason I like the wizard is it has a couple different specs, mainly Archon or blizzard for low MP xp farming, and the CMWW for high MP. But each build has many varients so there's a lot of custimizability. Also, Wizards are agruably the most OP class for Ubers, both solo and in group, since they can chain freeze Ghom/Rak and SB/Kulle so that they don't really do anything for the entire fight. What other class can solo MP10 with only 100k char sheet dps fully buffed?
I have all classes on 60 and 3/5 with Diablo dead and I have to say I find the Monk to be just right when it comes to balancing and utility. He can tank or deal huge damage, heal, give interesting buffs but is all in all a great challenge to master.
My mains a barb which I've played since the beginning, but I've also played DH, then monk, and now CM wizard.
Even though I'm not nearly as familiar with the build or play style of my wizard as I am with my barb I'm going to vote wizard. so far it's just much more enjoyable, especially in parties or public games.
I have all classes at 60, and with all of them I tried to finish the game. Except for DH, which by the start of Act 2 had its gear sold and is just standing there naked in my character selection screen doing nothing ever since. Did not enjoy it at all.
Started with Monks, which are cool, but my favorite is by far the WD, which is my main. Then Barb because it, for me, embodies what Diablo is all about. Monsters flying in the air is always fun.
I'd go for WD, if not Barb, if not Monk, if not Wiz, if not don't do it.