Well I just read your original post, cause I had it open while I was on a conference call. Then before replying, I hit refresh to see what everyone had replied. Heh... Ok, it's Alla, so I'll have a go at this from my own perspective. That of someone who has played pretty much all of what you're asking, but not on progression servers.
Necro is always fun to play. Fear mechanics let you be the god that you are until fear is essentially removed as a mechanic (summoning, etc). You have a powerful pet and powerful tools in your kit - let's not discount invis, invis to undead, and feign death in addition to the rest. This lets you move freely and abort, retry when needed. Sure there will be a ton of necros playing but you know you can still have fun with it. The group appeal of the necro is that most don't group. You bring a lot of dps to a group - smart necros can even pull for groups without a better puller.
Druids... solo machines from original to well into this progression. You have all the utility everyone wants either as a drive by buff, a group mate or a powerleveler. It's a dull class initially, a fun class in the midrange. My experience over level 50+ is dull, dull root rotting that I will never, ever play again. I've read posts by level capped druids that makes it seem that it gets better but I'll never play a root rotter again in any form. It's not fun, it's not interactive. Polar opposite is the enchanter soloing by charm. You know what I mean.
Enchanter... such awesome group utility that no group wants to even operate without you. Clarity and mez alone make you a key player always. You can revel in that role or go solo and charm for the non stop rush. I did it too and frankly don't know if I can sustain that kind of intensity again. If you can't though, you can always fall back and do your relatively sedate group chores of buffing, mezzing, etc.
SK... well that was always my original main, and I soloed him in original/vanilla (whatever) with the same crap gear we'll have all over again. I never died (ok, I died once when I was sitting next to the druid GM at the north karana port, but only because a damn bear attacked me and the druid GM then aggroed... grrrrr). I leveled at my own pace and I enjoyed every moment of it. This is by far the darkhorse because you will move slowly everywhere, but with the confidence that you are always in control. I never did Greenmist either, and am considering rolling an iksar SK when kunark unlocks to give it a try. My well considered caveat to playing a SK is that you know what a necro can do well. You will never be as good at what they can do (solo) in the early expansions. If a SK and a necro group, the SK will be the one with the downtime. That matters again.
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