I just realised I didn't play too much DOS games, there goes my list. (at least by now, I'm trying to play many missed classics).

Sim City 2000
Prince of Persia
Dark Forces
The Settlers II
Wing Commander
Doom
The Secret of Monkey Island
Monkey Island 2: Le Chuck's Revenge
The Dig
Quest for Glory I
Quest for Glory II
Quest for Glory III
Quest for Glory IV
Fallout
Zork III
Ultima IV
Dangerous Dave (the first one)
Tomb Raider
Cannon Fodder
Caesar II (PEBS ARE NEEDED)

Blood
Command & Conquer: Red Alert
Descent
Doom
Doom II: Hell on Earth
Duke Nukem 3D
Earthworm Jim
Earthworm Jim 2
Heretic
Leisure Suit Larry III
Leisure Suit Larry: Love for Sail!
MDK
Oddworld: Abe's Oddysee
Quake
Rayman
Realms of the Haunting
Sam & Max Hit the Road
Shadow Warrior
Star Wars: Dark Forces
Superfrog

That's a lot of games, I never even heard of. And I consider myself pretty well versed in the realms of Dos games.

Got some time to explore, what i must have been missing out on.

In no order:

1. The Secret of Monkey Island
2. Doom
3. Colonization
4. Prince of Persia
5. Indiana Jones & the Fate of Atlantis
6. Lemmings
7. Stunts
8. Sam & Max Hit the Road
9. Gabriel Knight Sins of the Fathers
10. Warcraft Orcs & Humans
11. Lands of Lore
12. Ultima Vii serpent isle
13. Sim City 2000
14. Links
15. Tetris
16 Worms
17. Loom
18. Theme Park
19. Another World
20. Alone in the Dark

1) Indiana jones and the fate of atlantis
2) Dangerous Dave
3) Bio Menace
4) Secret Agent
5) Doom
6) wolf 3d
7) Monkey Island 2
8) Prince of Persia
9) Sim City 2000
10) Duke Nukem 3d
11) Blood
12) Sky Roads
13) Commander keen
14) Another World
15) Full Throttle
16) The Dig
17) Zak McKracken
18) The last express
19) Raptor: Call Of The Shadows
20) Disney's Aladdin

It is hard not to check back on this post to see if i missed any titles

Fragile allegiance btw

certainly worth noting !

In no particular order. In many cases games of a series are together, or the places would fill fast.:

Apogee Jump'n'Run games (Keen, Cosmo, Secret Agent, Crystal Caves, Monuments of Mars, Duke Nukem, Bio Menace, Monster Bash, etc.)
Doom + Doom 2
Hexen
The Settlers 2
Heroes of Might & Magic 1+2

Clyde's Adventure
Sid Meier's Civilization + Colonization
Maniac Mansion 2: Day of the Tentacle
Indiana Jones 3+4 (Lucasfilm adventures)
The Secret of Monkey Island

Magic of Endoria
Warcraft 2
Alone in the Dark 2
Pizza Connection (aka Pizza Tycoon)
Trucking (German game by Steffen Datentechnik, 1995, apparently no info about it on the internet?)

Nomad
Transport Tycoon
XCOM 3
Fallout 1+2
Command & Conquer + Red Alert

Star Control 2

Then basically nothing, I'm not a nerd who makes lists.

I joke of course but you have to expect these responses from me.

EDIT: Alright, my list:

#1 Star Control 2
#2 Albion
#3 Syndicate
#4 Cannon Fodder 2
#5 The Settlers
#6 Gene Wars (don't judge me I just like the game)
#7 Baldies (GOG release, please!)
#8 Ascendancy
#9 One Must Fall: 2079
#10 Tomb Raider
#11 Magic Carpet 2
#12 DOOM
#13 Duke Nukem 2
#14 Ultima 7
#15 Fatal Racing
#16 Fragile Alliance (game of my childhood, so many good memories <3!)
#17 Tyrian
#18 Zone 66
#19 Inferno (I cheered so much when the GOG release appeared)
#20 Terminal Velocity

1: Doom
2: Alone In the Dark
3: Heretic
4: Raptor - The Call of the shadows (The Dos Version)
5: Quake
6: Warlords II
7: Warcraft II
8: Duke Nukem 2
9: Descent
10: Rayman
11: Extreme Pinball (GOG PLEASE!!!! :.( )
12: Hexen
13: Jazz Jack Rabbit
14: Blood
15: Tomb Raider
16: Prince of Persia
17: Xargon
18: Duke Nukem 3d
19: Fallout
20: Sam and Max

not in my top but I love them too

Rise of the Triad
Wolfenstein 3d
Gabriel Knight
Fullthrotle
Cosmo's Cosmic Adventure
Skunny
Too Many Geckos
The lost Vikings
In Search of Dr Riptide (GOG PLEASE!!!! :.( )
Magic Carpet
Commander Keen
Might and Magic III
Might and Magic IV and V
Abuse
Strife
Realms of the Haunting

If I tried to go through the DOS games I played to come up with a list, kept meaning to also post it and wasn't getting around to it, but I probably should if I don't want to have it be considered necroing.

20 is a large number though, and I can only talk of the games I played, or more exactly to those I know I played, since there are certainly at least a few I never remembered. If limited to one game per series or if multiple games in a series count as one, on first pass I came up with 15, some of those with caveats as well. If there's no such limitation, then 16. Then added the remaining four gradually. So there are a few that might be harder to justify, besides the probably obvious one. But I'll just list alphabetically:

Another World / Out of This World
Betrayal at Krondor
Caesar II
Sid Meier's Colonization
Elite Plus
Grand Prix II
Heroes of Might and Magic II
The Incredible Machine 2
Quest for Glory II
Quest for Glory III
The Lost Vikings
Lotus: The Ultimate Challenge
Master of Magic
The Settlers II
SimCity 2000
Stunts
Supaplex
Traffic Department 2192
Transport Tycoon Deluxe
WarCraft II: Beyond the Dark Portal

PS: This thread also reminded me of that "list of worthwhile DOS games".

It was really hard to reduce my list to 20, but here are the games I played the most during the DOS era. Some of these games had sequels which I loved just as much or more than others on the list, but I decided to leave them out to have more variety.

As a special mention, Maniac Mansion is one of my favorite games ever, but it's not on the list as I didn't have the DOS version back then, I played it on my Commodore 64.

The list is more or less ordered by year, but some may not be in the right order:

Alley Cat
Sopwith
Livingstone, I Presume?
NightHunter
North & South
SimCity
Street Rod
Prince of Persia
The Secret of Monkey Island
Another World
Drakkhen
SimAnt
Supaplex
Civilization
Alone in the Dark
The Legend of Kyrandia
X-COM: UFO Defense
Lemmings
Syndicate
Epic Pinball