Wolfenstien was pretty good when it was fairly new, but in the Last few years it has made me physically sick when trying to turn. Five minutes is the most I can take before I have to go lie down. I don't think I have this anymore.
Doom/Doom2: Played it no more than a week ago.
A bit with the DM-ARMY mod (All the monsters look like multiplayer guys, and shoot the weapons you would expect from a multiplayer game)
Then a bunch with my own deadly mini-mod. (Blood spatters have a 30% chance of exploding like a rocket to represent critical hits.

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Duke Nukem 3D: Haven't played it since we misplaced our serial cable, but with a mod and custom map, still good fun
The mod we always use for challenge gives the basic enemies a 40% chance of spawning 2 more when they die, and a small chance of spawning one of the bigger baddies as well.
Firing rockets into a crowd of guys can actually make things worse
However, they always drop something decent when they die, and sometimes even drop powerful things too. As well, the player nearest to an enemy who dies gets +1 health.
The 4-dimensional map capability of this game is extremely cool!
You could have an open field, with a twilight-zone style door standing all alone in it. Walk through the door, and suddenly you find yourself in the same open field, but at night, and with monsters! Walk around the door instead, and its still daytime
Starcraft is pretty good. Beautiful graphics, and decent gameplay, but not terribly flexible.
I did manage a nice mod to choke off the "rushes" strategy using a custom map. The way "Small Squad Tactics" works is:
1) the minerals available on the map are slashed to a fraction of the original stashes.
2) Whenever you have more gas than minerals, they are traded at a 2:1 ratio.
Since gas vents merely get "depleted" and never run totally dry, you need to capture the vents in order to continue to expand.
With the thin minerals, you need to capture those vents and defend your base with as few units as will do the job. As well, you want to protect the lives of all your units. If you pull out when crippled instead of fighting to the death, you'll save immense amounts of money and time by either doing repairs, recharging shields or healing instead of building a whole new unit with your meager funds.
Going back to the BBS days, my favorite:
Land of Devastation!
Extremely moddable; there is even a programming language to add new stuff to the game!
I've added convenience devices that scan the ground as you walk, and automatically pick up items of value. I've put in all kinds of monsters from other games... Protoss Archons are especially sought after, often in vain attempts to steal their shields, which have the property of self-recharging over time (which I coded in myself

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In VGA mode on the local computer, it really looks good too.