Do old games with modern ports count? I play the original Doom and X-Com all the time, but I use GZDoom and OpenXcom because I prefer having a few modern features. I also replay the original Deus Ex and Half Life 1 frequently, but I also mod them for modern systems. FPSes are the one genre where I need a more modern experience, such as widescreen graphics and better mouse support.

For a pure DOS experience, my all-time favorite is Stunts, which I never get bored with.

If I'm setting up a new DOS or Windows 9x build, I always test it out using the classic shareware games, such as Jill of the Jungle and Commander Keen. I just use the shareware versions from the 90s.

A few other DOS era games that I keep returning to include the following in no particular order:

EGA Trek
Solar Winds
Sim City
NHL Hockey
4D Boxing
Empire Deluxe
Space Quest I, II, III, IV

For Windows, I like to revisit the standard 3.1 games every now and then, such as Ski Free. That's more of a nostalgia thing though.

Probably the top would be Fallout 1 and 2 , I've beaten them a bunch and replay them every so often.

Ofc you cant go wrong with Doom plus its great for testing your Roland Sound Canvas out.

I can vouch for Epic pinball but also Jazz jackrabbit which yeah are pretty good ultrasound tests.

Simcity 2000 is a really great game to chill out to and relax and forget your worries and just build a city.

Dune 2 is pretty fun to hop into and have a game or two plus the intro never gets old.

Ultima underworld because I hop in usually get into an unwinnable fight only to leave it for later (hopefully ill finish it someday).

Daggerfall because I still somehow haven't done enough rng quests to progress the main storyline which is kinda annoying.

I don't replay all that much these days. My backlog is just way too long. Also, to me everything is retro that is at least voting age, regardless how modern it is. So currently up to Q3/2005.
That would make the my most (re-)played games:

Quake 3 Arena
Half-Life 2
Quake 4
Call of Duty
Prince of Persia: The Sands of Time
Command & Conquer: Generals
World War III: Black Gold
Codename: Panzers - Phase 1
Grand Theft Auto: Vice City
Need for Speed: Underground 2

Most of these are still in my performance test rotation and I've beat them at least half a dozen times or played skirmishes/quick races for at least a decade.

If you mean titles that are good for a quick gaming session, for me, those would be:

• Tyrian - very enjoyable bullet hell shooter
• Doom 1 & 2 - sometimes, I just drop in for a quick run through a couple of levels
• Duke 3D - same as Doom
• Master of Orion 2 - probably the best 4x strategy game
• Transport Tycoon - always fun to revisit
• Unreal Tournament '99 - I occasionally run a few botmatches for some classic FPS action

Other games that I've played through several times include Gothic 1 & 2 and Risen. These definitively aren't suitable for a quick run, but they do offer a lot of replay value, which has me coming back from time to time. It's particularly fun to join different factions and see how it affects the story and quests that are available to you.

For me it would be:

Civilization
Doom (Ultimate)
Eye of the Beholder 1/2
Quake
Settlers 1/2
Sim City 2000
XCOM 1/2

These games I keep going back to..

on real retro hardware (intel 440bx, ausus p2b 400 mhz and a riva card)

Jagged Alliance 2
Command and Conquer Tiberium Sun
Commandos
Mdk
Quake

These games run well.

on my modern day rig

Ryzen 5 3600 and Geforce 3070 RTX card

i also play some retro games that are to modern for my current retro rig. I will play these games too on original hardware as soon as I have my second more powerful rig.

Return to Castle Wolfenstein
Quake 3 Arena
Unreal Tournament
Max Payne
Mafia