The most impressive class system I've seen in an RPG is in 1992's Wizardry VII: Crusaders of the Dark Savant (as well as its less well-known predecessor, Bane Of The Cosmic Forge). What initially appears to be a straightforward, rigid D&D-style system is given massive depth by two things: the importance of racial base stats in determining class, and a dynamic system that allows for both massive improvement and possible disaster. It's possibly the most in-depth system I've seen in any RPG, and one that I wish was more well-known.

[...] This is the greatest strength of the Wizardry VII engine. It allows you to develop your characters in any number of different fashions. The most normal method may be to start with six fairly simple characters – two fighters, a thief, and three magic-users, and convert them all permanently to more powerful classes like Samurai and Lords when they can change. Or you can build a party of characters designed to constantly switch to and from certain prestige classes like the Valkyrie or Ranger which keep most or all statistics high as well as fairly even. You could never change character classes during the entire game. Some people have even found it possible to play through the game with a single character!

I've never found a game with as deep or effective of a class system as Wizardry VII. The closest comparison is the Final Fantasy V/Tactics system, which allows you to maintain the skills of your old class after switching to a new one, although it lacks the complexity or potential for failure of Wizardry VII. Even the more accessible sequel, Wizardry VIII (2001), did away with critical components of this system. Wizardry VI/VII stand alone with the deepest, most powerful, and best class systems in RPG history. They're difficult, but well worth the trouble.​