I'd vote Necromancer, with Druid or Shaman as close seconds. All three can:
- solo with virtually no equipment (I leveled an almost naked druid to 45 without issue)
- are useful in groups (although Druids a bit less so)
- have ways of farming plat at higher levels (necros and shaman can solo valuable loot and druids can join dial-a-port)
- cover a broad spectrum of play: all three can heal, DoT, nuke, use pets, and charm (although Shamans only briefly)
All three are great up until 45/50-ish. At that point however Druids start feeling less powerful and desirable, while Shaman have to acquire two very expensive items (their epic and Torpor).
Enchanters are a good 4th option. They're not gear-dependent, they're pretty well-rounded and they are desired both in solo and in groups. However, they have a more unique play-style and can't heal, so they're maybe not quite as good for "re-learning the game". And Bard, Mage, Cleric, and others are all viable options too, they're just less self-sufficient and/or more one-dimensional than the above classes.