I'm willing to stretch the definition a little. The first level for an episode I feel like should count for this list.
Here's my personal ranking:
(Also, these are based off my experience, usually on "hurt me plenty" difficulty)
1. TNT: Yeah, I choose TNT for #1, and I don't exactly know why? It's not too much, 1 secret, and 36 monsters. But maybe I just like the simple levels. Also, beserk pack.
2. E4M1: This one is a nice challenge, took me a while. It's not too special, but it works as the first level of the "extra levels."
3. Doom 2: Not special, but I like it. I like how open it feels, the secrets, and the secrets' rewards. It's just a good level.
4. E3M1: I love the athstetic of this one, it's very beuatiful. And I can excuse the cacodemon, it's episode 3, you should be used to them by now. Simple, short, sweet. Just like cereal.
5. E2M1: This one... is tricky personally. I like it, but it's, kind of, boring. I don't know. It's a fine challenge though, I'll give it that.
6. E1M1: If E2M1 is boring, this level is the equivalent of watching paint dry. I just don't care for episode one personally. And this level doesn't help matters.
7. Plutonia: >:(
I would love to hear your rankings and opinions on these classics!