Copypasting my comment from a Doom thread for context:
I finished Doom, Doom II, and Thy Flesh Consumed in that order over the past two weeks.
Disclaimer: I played all of them in Ultra Violence, with only Doom being played using mouselook and the jump button (as I didn't know it wasn't a thing... but I already had my suspicions in a few levels).
I thought that Doom was much more enjoyable than Doom II. Probably because of the amount of enemies the latter throws at you. I used a lot of savescumming in the latter, in the sense of "save before a hard area, save after being done with it no matter my remaining health". It's possible Doom II was also much harder for me as I had to get used to no mouselook. Indeed the first levels were very very clunky for me to play through, but as I play Evilution now I feel like I'm much more competent. I was able to easily beat the first three levels of the original game in Ultra Violence without a single death (until an unexpected swarm killed me in the fourth level I believe).
Doom II's levels felt less inspired I think. Like it wanted to be harder but without the creativity of Evilution (the levels I've played so far at least). Overall I agree with the OP that the game's are short and I took my time playing them. Probably because in that one hour of gameplay I get so much more thrill than I would get from six or seven hours in other games. Doom is so fast paced that it is a nice change of pace from games where I would walk for minutes on end endlessly thinking about my life (instead of, you know, thinking about the game I'm playing). The shooting is so satisfying and it does make me question my choices in other games (like manual reloading).
Overall I'm really glad I played through these. It's something I had in my "to play" list for a long time, and something I thought I would never get around to doing.
Now I can also bask in the memes.
UPDATE
So I'm around level 23 in Evilution.
I have to say I'm not enjoying this WAD. Mostly because the levels are BIG in the sense of "spread out", as opposed to BIG in the sense of "cramped, but large". I don't know if that gets my point across.
I also heavily dislike the amount of chaingunners. The levels start getting pretty predictable as well, with an item usually being an indicator of "grab me and watch the walls around you collapse to reveal 30 chaingunners".
But it's good in the sense that this helps me realize that what makes or breaks Doom is level design. Which should be fairly obvious, right? However, I've been comparing Doom to my most played RPGs (which I won't mention in case of butthurt, but everyone who knows just a little about me will know which ones they are), and it made me realize that those games just aren't particularly fun after a while, and no level design would change that because their core gameplay is fundamentally flawed.
I'm really itching to try the neat total conversions I've seen around, particularly Ashes and Aliens: Eradication, which look atmospheric as fuck.