Ritualist, on account that I greatly enjoy playing versatile classes. Ritualist is likely the most diverse class in terms of what and how much it can bring to the table, so, ergo, I enjoy it. There's just something appealing about being able to deal damage, heal, buff, and run all with one character. Of course, I'm talking mainly about PvP, since you're most likely insane if you run any caster in PvE that isn't a Necro or Monk.
I suppose Paragaon comes in a close second, as, once again, they're quite versatile, bringing damage, buffs, healing, and more to the table. Oh, and unlike Rits, there's a reason to play them in PvE.