Favourite class at the moment depends on two things: my mood at the time, and what I've played a lot of lately including as NPCs in my own game.
I used to play a lot of Fighters (some of them are still around) and really enjoy them but I've kinda done what I can with them. Still, if I want to switch my brain off and just go gonzo, gimme a Fighter any time.
I really like Illusionists (the 1e version) because of the flexibility of what their illusions can do; but I've also played them to death over the years.
More recently I've been playing a number of MUs, largely because a) they seem to be the ones who last. I like them, but it's getting near time to move on. That said, I just put one of my MUs back into the field as that group's current mission seems tailor-made for mages and other literate types (we're searching for a lost library, potentially with magical writings in it).
I've never run a good old-fashioned Cleric (other than a few one-hit wonders) until recently, and am still discovering what he's got going for him. And I've never had a Thief last any length of time but I've got one now in a different group (same campaign) who, while still kind of in the "hang on and hope" phase as she's low in both level and wealth relative to her party, has a lot of potential.
My true favourite, though, is probably the heavy-tank Ranger archetype - very possible in 1e but really hard to do in later editions without multiclassing. I really oughta boot one of those up sometime...been many a year since I played one as a PC.