I'm no minmaxer, I'm pretty vague about talents and I don't have awesome equipment, so take the following as you wish
The chars I've played up past normal have been wiz (awesome) ranger (fun so far, high end of nightmare), SK (variable!!), mage (fun times), rogue (STAB) and currently working on shaman, who's proving pretty cool.
They all play differently and scratch a different sort of itch depending on what you feel like for a session of play.
I agree the wizard feels like it has the best range of abilities and would be really OP if it didn't get laminated to the pavement by multiples.
Ranger is hilarious for speed and number fountain but I've found it a bit fragile.
SK I'm struggling a bit to do significant damage, but reasonably tanky with a few interesting quirks.
Rogue can suffer with awkward pulls and unlucky status hits but on the whole seems to be pretty robust and fun/fast to play.
Mage is a bit of a different puzzle, feels a bit fragile to me with limited healing but you need to think about types of pet and elemental switcheroo, which makes a change.
Liking the shaman and again it's a different style leaning heavily on dots and pet and not so much on direct damage (so far anyway - L40+) - bit more strategic.
Can't cope with monk or bard
Perhaps they're awesome at higher levels but you have to want to play them enough to get them up there, and that ain't happening for me.... Also bard/chanter apparently rely on charm pets, and I'm scarred enough by years of playing a chanter in EQ that I'm reluctant to go there
I've got a cleric and druid in mid twenties who I've got my eye on next.
Altaholic? Moi?