I dislike the non-class-based approach that D&D 3.0 (and later) turned classes into.

Properly, a class is a socio-economic position. In original D&D, a first-level Fighting Man1 was a “Veteran”. He grew up learning to fight; it was his entire lifestyle. Similarly, a first level magic-user grew up being trained in magic. You can’t just “pick up a level” in a socio-economic class, for the same reason I can't just "pick up a level" in Olympic athlete or millionaire.

1 Yes, that was the term. The character races were Men, Elves, Dwarves, and Hobbits. It's worth noting that all players I met at this time were guys.

My favorite true class-based system is Flashing Blades – role-playing in France in the time of the musketeers. The classes are Noble, Gentleman2, Soldier, and Rogue. They affected what skills were available, and how you learned to fight (fencing academy, barracks training, school of hard knocks).

2 See Fighting Man. When I ran the game, I invented the character class Actor, because in the 17th century, actresses were fairly autonomous.

I wish the non-class-based learning paths in 3.5 were named something else. [“My character has two levels on the Fighter Path, 4 on the Ranger path, and 6 on the Horizon Walker path.”] I like the game, just dislike that abused terminology.

My favorite games are:
Original D&D
Flashing Blades
Champions (3rd – 5th level)
TOON
D&D 3.5e
These are in chronological order, since I can’t really give them a preference order. I like them for very different things.

The looseness of original D&D made it a perfect game for imaginative people. You didn’t have a search check. You had to discuss how to search the room. Did you remember to look at the bottom of each bookshelf? Its biggest strength (and biggest weakness) was that it isn’t a game; it’s a set of pamphlets to help you build a game. [People who started with this game are usually far more comfortable with DM judgment calls instead of exact rules, since that’s what this game is.]

Flashing Blades has a minimal system, just enough that you can focus on rapiers and court intrigue, not on mechanics. It’s as immersive as any game I’ve played.

Champions allows near-infinite flexibility to design characters. Its only flaw (arithmetic-based character design) is no problem for me – a math Ph.D.

TOON is the exactly correct madhouse for simulating cartoon characters. If a character tries to do something clearly impossible (an elephant tries to go through a mousehole), you roll on his Smarts. If you fail the roll, you succeed at the task.

And 3.5e has a richness of detail and option that allows a huge ability to build unique characters.

So I don’t prefer class-based or classless designs. I don’t prefer old games or new games. I prefer systems that allow you to do what the system is supposed to do, within the limits of it being something I want to do.