I have to say the 3 of the holy trilogy are close to each other, the 3 of them has great level design. With that said:
Duke3D is the most accesible for anyone, the 3rd Difficult "Damn I'm Good"(I don't care the 4th, since the only difference is the respawns, which I hate like in Doom) is both, challenging and fun and the combat feels good for the must part. Duke has one of the worst shotgun of old fps, and feels underwhelming, it is ok but less fun than must other fps.
Blood is a HUGE step forward in every corner. The combat (even tho is a litle bit too "floaty") is incredible fun and adictive. Weapons are really cool (not you voodoo doll) and fun to use and is super challenging in Well Done! (3rd difficult). I tried "Ultra Crispy" but the number of hitscaners in the end of the 1st episode made me quit.
This game does not hold your hand at any point and is not for everyone, probably my favourite but really close to SW.
Shadow Warrior was my second game of the holy trilogy, and I was blown up on how ramped this was from Duke3D. The combat was brutal, lots of hitscaners and Shadow Warriors" camuflaged in the background with those oneshot Guardian heads of theirs. I really love every weapon in this game, specially the Grenade Launcher.
I loved for the most part every aspect of this game, even "No Pain, No Gain" (the 4th difficult) was brutal but doable. close call to Blood.
I won't count Ion Fury since is newer and the game is super cool and polished.