What are the most technically impressive/biggest overhaul DOOM wads/mods?

Like, they aren't necessarily the best (though they can still be great) but they changed so much to the DOOM formula, that they can be considered their own game, or DOOM WADS that push the boundaries on what can be done on the engine.

Wads:

Comatose - "A limit of sidedefs in LR format is achieved here, so it is not possible to draw at least one new line at all".

Mods:

Lithium - "gameplay mod that features an upgrades system, currency, two new classes with full weapon sets, an inventory system, deadlier enemies, monster mod and multiplayer compatibility".

DoomRL Arsenal has over 200 weapons.

The Guncaster adds an overwhelming amount of new content.

Doom Tournament - "UT99 themed weapon and item replacement (with some extras)"

Total conversions:

Batman Doom transforms the game radically.

REKKR "vanilla TC set in a fantasy world", "For use with doom.wad (Ultimate Doom 1.9)."

There are more, of course.

Total Chaos, regardless of what you think of the gameplay, is quite likely the most technically impressive overhaul of Doom we're likely to see in a good long while.  Other than the orthogonal movement of the enemies, it's almost completely unrecognizable as GZDoom.

Oh, I almost forgot Mr. Friendly. It effectively inverts the Doom formula, while still being a Doom mod.

I'd probably think of TC's (total conversions) as the ones that, and i quote, have "changed so much to the DOOM formula, that they can be considered their own game".

Don't get me wrong. There are also alot of PWAD's/mod's out there that are extremely impressive that aren't considered TC's but i'm just going to list some TC's for you that i can think of off the top of my head starting with the "classics".

• Aliens TC - An oldie but a goody. Also the first TC for Doom (i believe). I've played it & it's pretty damn good imo.

• Army of Darkness TC - Another oldie. Based off of The Evil Dead: Army of Darkness movie. This is the Doom 1 version but there is a Doom II compatible version as well. I haven't actually played this so i don't know how good it is but it is quite popular (or rather it used to be anyways).

• Batman Doom - Another oldie. A Batman based TC. The links are at the bottom as there are different versions for different source ports. I haven't played this one yet either so i can't comment on it. 

• Operation Body Count - (link is at the bottom) A somewhat newer one. I found out about this one recently so i'll mention it. Basically a remake of the old original game with a few changes i believe.

• Action Doom - I mean, this one is just crazy. It definitely changes the way you play Doom by incorporating a classic "shmup" style of play which is probably why i'm quite bad at it but it's still fun nonetheless haha!

• Total Chaos - Like @Bauul said this one is probably the most impressive looking TC, technically speaking, as well as the one that play's the least like Doom.

• REKKR - An impressive looking new vanilla TC for The Ultimate Doom. I haven't tried this one either but i plan on it soon.

• The Adventures of Square - How can i not mention @Jimmy (as well as others') impressive standalone game that uses the GZdoom engine? If you haven't played this, you definitely need too as well as it's sequel (EP 2).

Well i could go on & on as there are a number of great TC's that have all pushed the boundaries of the Doom engine as well as the limits of source ports but i think this small list is sufficient for starters ;)

The Golden Souls series and Castlevania: Simon's Destiny succeed at turning Doom into a platformer while still keeping it as a FPS.
Hideous Destructor succeed at turning Doom into, basically, Arma. (or any tactical shooters out there)

PokeDoom succeed (kinda?) at porting Pokemon into Doom. The project got abandoned due to very unfortunate reasons, but still worth checking out.

Blade of Agony is basically an unofficial sequel to Wolf3D.
The Crystal Maze did a pretty good job at making you solving puzzles instead of mindless slaughtering, for the most parts. Same with Skulldash ED, but instead of solving puzzles, you are collecting coins as fast as possble.
GMOTA turns Doom into an arcadey, medieval-themed game.

Smooth Doom and its Dehacked variants, well, make Doom smoother. 

Also, here are some mods featuring modern features in Doom.
 

Hunter's Moon - "the single-player campaign that Quake III Arena never had".

Netronian Chaos - completely original roster of enemies and weapons.

Rebel Rumble - "This mod is all about some arcade Shmups like Total Carnage, Smash TV and Raiden Series."

Reelism - "High Chaos on a Grand Scale!". It's huge.

Ashes 2063 TC - post-apocalyptic TC.

Hocusdoom - "What would happen if Hocus Pocus was a fps like Doom, rather than a sidescroller?"

5 minutes ago, m8f said:

Hunter's Moon - "the single-player campaign that Quake III Arena never had".

Netronian Chaos - completely original roster of enemies and weapons.

Rebel Rumble - "This mod is all about some arcade Shmups like Total Carnage, Smash TV and Raiden Series."

Reelism - "High Chaos on a Grand Scale!". It's huge.

Ashes 2063 TC - post-apocalyptic TC.

Hocusdoom - "What would happen if Hocus Pocus was a fps like Doom, rather than a sidescroller?"

Don't forget to also include your toolbox ;)

Simpsons Doom.

And jDoom

Some of the technically advanced/impressive/boundary pushing GZdoom mods I am aware of are:

Total Chaos: Most technically impressive doom mod (despite me not being a fan of its gameplay). It is almost unrecgonizable as GZdoom mod and looks more like a Half Life 2 mod.

Hunter's Moon: Very advanced mod that is meant as a singleplayer campaign of sorts for Quake 3.

Blade of Agony: Very advanced mod that is a sort of sequel of Wolf 3d and has MOHAA type feel to it.

Brutal Doom: Yes, yes I know it is considered a sin to mention this mod here. But v21 is loaded with features. BD now has vehicles, level enhancement system, body grabbing, 15 something weapons, adjustable gore, demon transformation. Most of these features can be toggled on/off.

Ashes 2063: Very advanced post apocalyptic TC mod

Hideous Destructor: Turns dooms into a very complex tactical shooter. A bit like Arma.

Hellshots Golf: "As far removed from running and gunning and shooting demons as you can get."