Inferno has over time aged the best in my opinion. Not in terms of looks (it's a Sandy episode after all, and I'm still operating under the theory that Petersen was colour-blind) but in terms of overall, bigger-picture vision and experimental scope. Gonna do a quick list. (Morgan Freeman voice: "it turned out to actually not be a quick list")
M1: "bawwwww, there's no ammo! It's a pistol grindfest!"
Actually, fucking run! You're in the thick of it here, and that opening ambush (which would've been cruel as hell for non-strafing keyboard players back in the day, let's keep things in this perspective) demand you get in there even thicker. It's a short map, but certain touches (demons in the narrow corridor blocking the path and making the risk of fled-from monsters flanking you all too high) are genius.
M2: The One Shaped Like The Hand
I've always loved how this one feels like you're out in the wilderness. It's a Sandybox alright, with the simple goal of locating the blue key and exiting with it -- whilst there are monsters tucked in all directions, keeping you on your guard. I love the atmosphere above all; that it's shaped like a hand is just an automap oddity to me; it's a living breathing ecosystem here, a quality found in a lot of DOOM maps I love.
M3: "it's pan-duh-MOE-nee-um, not panda-money-YUM"
One of the weaker entries, I must admit. But one that offers probably double the optional content as it does mandatory DOOMing, and harks back thematically to episode 2's hell taking over tech flavour.
M4: So Get Out Your Seat And Jump Around. Jump Around.
Another one that's an automap treat (a set of human internals) and largely linear, with some standout moments. Sandy gets his puzzle head on with this one in a few areas, which is a nice change of pace. Environmental storytelling abound in the inaccessible window area with all the bodies tied up to pillars; all topped off with an ending setpiece ambush that serves more a cinematic purpose with the provided invulnerability sphere than anything; running through the corridors and waking up monsters from all non-boss tiers on the way, really says to the player "you're in deep now - you're fucked".
M5: This Would Make An Awesome Band Name
Look, alright, the teleporter you need is glowing. Pain in the ass sorted. More Sandy puzzling and actually some of the more believable architecture from the man; at least in the opening area. The post-teleport ambush setpiece, the pinky-punching berzerk section, the switches behind crushers; Petersen seemingly experimented with different configurations of challenge here. Some hit, some miss. But it's a very distinct map that couldn't have fit in either of the first two episodes due to its oddness.
M6: Wait, isn't the real Erebus actually fucking freezing?
For my money, the very finest map in the entirety of Ultimate Doom. It's the archetypal Sandybox, a whole bunch of scattered challenges of different natures with secrets aplenty and a simple-but-hitherto-obscured goal; something I love personally is when a sprawling adventure of a map reveals itself to be functionally very simple, giving me as a player the option to either get it over with or actually fool around popping demon heads. I think also, this is one of the more hellish maps from an aesthetic standpoint, some of the texturing is just uncomfortable. It lends this map a very surreal quality, contrasted against vaguely-recognisable structures dotted here and there. Also, this map is my entire argument for SR40 being an intended 'canon' gameplay technique, as it allows access to the secret map without rocketing ones own face against a wall.
M7: How Low Can You Go?
Hell is having to wade through damaging floors whilst a horde of pinkies want to eat your face. Much has been said about the entire lack of sky making this subterranean affair feel claustrophobic; add to this the 'timed' pressure it gets from there being only a finite amount of radsuits (although it has to be said, there are more than enough in reality; another showboaty 'cinematic' thing Sandy has employed here IMO). A lot of Inferno feels 'otherworldly', this is the map that to me feels the most 'other-dimension-ly', as all the teleporting back and forth to areas overlooking other recognisable areas produces a feeling of disorientation; never enough to totally confuse, but disorient nonetheless. Like most of the latter E3, combat is dialled down somewhat here in favour of the pure atmosphere - a thing I'd wager heavily comes as a result of Sandy's experience with D&D and similar role-playing games.
M8: ...not to mention Dat
Totally underwhelming in 2018, but back when imagine the pain of playing with a keyboard, unaware the dedicated sidestep keys exist, possibly in low detail or a reduced-size window, stepping forward and having this motherfucker absolutely wail at you before sapping your health to nothing just because it can see you. You'd abuse that 'alt' key like it was nothing, carefully ducking back and forth from behind the center structure (as a bold and frankly genius move, if you get curious and enter the structure for the added armour and plasma rifle, you get goodies but pay for it with reduced cover). And at that, you're being flanked by a few other goons - deal with them, or keep the pressure on the spider? Tough call. Obviously this doesn't translate very well to modern FPS skills, but I'm taking this in the context of when it was created. Nobody had seen anything like DOOM.
M9: A Wrong Turn
Let's say you're playing continuously. With a keyboard and are unaware of those strafe keys. You've slogged through every level up to Mt. Erebus, and went through 'an' exit, being unable to find the regular one (which is sorta tucked away in a non-obvious place). You're greeted with the very same visage that opened the episode. FUCK, WHAT DID I DO WRONG? It's a bait-and-switch, of course, and hardly unpredictable right now. But back when, as you make your way; much more easily this time, with six missions' complement of weapons and ammo; to the exit... and it's not over. Everything lowers, and there's the dreaded roar you remember from the denouement of the second episode - The Cyberdemon. This would've been the coolest thing ever. Once.
Inferno is hell, through and through. It may be easy as shit with yer WASD and mouse-turning and foreknowledge of the layouts, but in the context of its 1993 release when a lot of this was breaking ground with new concepts, it was a great episode. And I say it's aged the best because it's still a good romp to run through with little pressure, and take in the Sandy-scenery.