Like I said, I got most of my stuff from gaming magazines before I had Internet, and I almost never saw any Quake mods on their CDs.
On the other hand, I was rather young when Quake was "a thing" and just started with PC gaming. I found out about these magazines a few years later.
I only worked on mods for UT and the original Jedi Knight, and I've only heard second-hand that UT and Quake (3? 2?) were easier to mod than HL1, so I might be wrong.
And compared to Jedi Knight and its only editor(s) being user-made, all these games were easy to mod!
Team Fortress, Quake Chess, AirQuake, ThreeWaveCTF, Xonotic (kinda, sort-of)
Think of three Half-Life mods, as fast as you can:
TFC, Counter-Strike, Natural Selection, Sven Co-op, Science & Industry
Think of three Quake 3 mods, as quickly as possible:
OSP, CPMA, ThreeWave (again), OpenArena
Think of three Doom mods/mappacks, fast:
Memento Mori, Brutal Doom, Batman Doom, Russian Overkill, The Sky May Be, Tech Gone Bad, and so many more
Think of three Minecraft mods, as fast as you can:
Thaumcraft, IC2, Buildcraft, Forge, FMP, Thermal Expansion, Better Than Wolves, RedPower, NEI, Ars Magica, Applied Energistics, Aether Mod, Tinker's Construct. You want more?
Think of three UT mods:
Ummm... Errrr... Ummm... If you count mappacks, somebody might have made a version of Aerowalk for it?
Now, that's just me, but I doubt others would have dissimilar responses.
It simply delivered one of the best modding possibilities ever.
You bought HL and every week you could buy a gaming magazine with 5 new mods. Most of them crap, for sure, but it felt like paying once and getting new stuff all the time.
I never had a game installed for so long and played the "main game" for such a short time.
HL², on the other hand, I played for the story only, never installed any mods or something, but maybe I was already to old to run after this half finished games, haha.