Erik Tenkar brought this idea up:
What is the most underused A(D&D) class?
Good question. In his experience, it was clerics.
But in mine, we had a lot of clerics. My cousin used to run Cleric after Cleric after Cleric. So much so that "plate armored front line cleric with mace" was kind of a thing in our games. He ran a lot of Thieves, too. We had Fighters, druids (the Dru the Druid XIV jokes I make aren't jokes), Magic-users, Illusionists (mostly of the gnome Illusionist/Thief type, but I remember a single classed human Illusionist, too), Rangers. I don't remember any Paladins back in the Players Handbook days, probably because elementary school kids don't like the idea of magic item restrictions.
Once Unearthed Arcana came out, we added Cavaliers, Cavalier-based Paladins, Barbarians, and more.
We used the Monks from Best of Dragon Vol. 3, too, and they were a lot of fun (even ask El Murik's player, he ran one.) We didn't use the ones from the PHB, because it was hard to roll one up and they sucked so badly at level 1 that no one wanted to be one. I've heard tell they don't suck, but AC 10, d4 HP, and no special powers for a combatant would mean instant death in my ruthless everything-is-a-fight AD&D gaming days. At least the newer monks could mix it up from level 1.
So underused? Assassins. I can't remember any assassins. We didn't play evil groups, and assassins aren't that good unless you're evil (and even then, if you use the PHB rules about poison, your best weapon is dangerous to you, too). Assassins were NPCs.
How about race, eh?
Back in my D&D days, we had a lot of elves, half-elves, dwarves, and the occasional gnome. Almost no halflings - maybe even no halflings - I can't remember any offhand. More than humans in most cases. We didn't play that long, so the front-loaded power boost of being a non-human multi-classed character was too much to pass up. We didn't get any half-orcs, though. So, no halflings, no half-orcs, and a lot of elves.
Tomorrow I'll post on the same subject, except for GURPS - so hold those GURPS DF template comments for a day. :)