I say B and C about equal, with a small amount of A. While I think B modules are better for DMing and are more worthwhile purchases overall, I think it's the C modules (T1-4, A1-4, GDQ1-7) that were classically epic, and A/C modules like Rise of the Runelords that are more modern classics.
The thing is, modules are often crap. Or they don't work with your group. Or they don't work in your campaign setting. But even when the modules are crap, there's usually some gems in there. The map is cool, or there's one encounter that's got a good design, or the hook is really effective, or there's some good idea in there. And sometimes, the modules are just great! When they're bad, you can use them to hook a campaign, or bridge a campaign, or run a one shot, or a side dungeon, or whatever. When they're really good, they can set you up for a good time.
So much of what I learned DMing was just reading modules and deciding what I liked and what I didn't.
This is why I miss Dungeon magazine so much.