REGARDING QUAKE:
Keep in mind of the divide between QW and NQ mods. QW are usually clan pandering multiplayer-only stuff, NQ is the common kind that a normal player would play. Some engines handle both (DP, FTEQW)
Zerstorer and Beyond Belief are mandatory playing.
Prodigy Special Edition is nice too (Dario Casali)
If you love Hideo Kojima you might like Nehahra
YPOD is your obligatory Doom-In-Quake!!!!! conversion. The only one that made it to reasonable maturity
A nice multiplayer mod Zdoom modders could learn/get inspired from is the original Pain Keep (not no silly crufty repacks). It has some nice levels, decent atmosphere, and some good weapons and tools.
Killer Quake Pack (KQP220Z.zip) was also another defacto multiplayer mod for trying to have a stable and huge variance in weapons.
The "AEoD" is Super Duper Quake and it's actually still being updated after existing for 17 years.
Ultimate Quake is worth a look at if you wanted to see how much feature creep stock quake can handle ina progs.dat. it's nuts and it got Mike Gummelt a job at Raven...
which he still has
And because someone will ask, there is no "brutal quake". there's however navy seals and usmc (which rips off navy seals) that try to bring militaristic firearm realism to quake
back in 1997 before it was cool. and the gore stuff are done time and time again then, so many mods with buggy 'solid corpses' and additional gib spewing that chokes the engine carelessly.
there are no essel equivalents
Avoid "HD" packs like the plague.
The mapping community is still very alive and is somewhat purist (limit removing maps designed for id1 mostly)
Modding scene is DEAD and/or pretentious, all you'll get are kids that want to recreate commercial games for some PSP port. The golden days has been the 1997 idgames2/ age (ended when Valve picked the best staples out and hired them to work on Half-Life).
The laundry list of source ports and their roles:
Darkplaces for modern general usage, new mods are mostly developed for this with its csqc/menuqc features in mind
Ditto for FTEQW.
DirectQ is the only port that faithfully recreates the software renderer's effects in pixel shader form. It's a very decent port to use on Intel HD at high framerates as well and is similar to quakespasm in map compatibility.
Mappers nowadays swear by Quakespasm because of its similar fixings to the renderer (up to software quake levels) and can nicely represent quake enough to satisfy the purist community. similar to glprboom
Engoo gets a mention for trying to be Zdoom. It is software rendered and tries to bash in very truecolory things into a 8bit buffer.
Other source ports are either Quakeworld (with physics you don't want in a sp/coop game) and or early doomsday-tier "programmer's fun with effects" stuff that aren't cut for general use.
and fortunately none of these require OpenGL 4 or a recent Nvidia card to run or even hit triple/quadruple digit framerates
