When the full game releases, I'll update this, but as of right now the only game we know is playable in SF6 is Street Fighter II.

-Space Harrier
-Fantasy Zone
-Super Hang-On
-Fighting Vipers
-Sonic the Fighters
-Motor Raid
-Virtua Fighter 5: Final Showdown

Includes several classic Atari games from Activision. This list is from the Call of Duty Wiki though I'm unsure if it's entirely accurate:

-Barnstorming
-Boxing
-Chopper Command
-Enduro
-Fishing Derby
-Grand Prix
-Kaboom!
-Pitfall!
-Pitfall II: Lost Caverns
-River Raid
-Robot Tank
-Cosmic Commuter
-Skiing
-Ice Hockey
-Keystone Kapers
-Seaquest
-Spider Fighter
-Demon Attack
-Plaque Attack

There is also an easter egg where you can play Zork I: The Great Underground Empire.

Classic Doom and Doom II are unlockable.

-Space Harrier
-Fantasy Zone
-OutRun
-Super Hang-On
-Virtua Fighter 2
-Virtua Fighter 5: Final Showdown

The PS1 version can be unlocked

-Space Harrier
-Fantasy Zone
-Fighting Vipers
-Motor Raid
-Virtua Fighter 5: Final Showdown
-Puyo Puyo
-Virtua Fighter 2 instead of Puyo Puyo in some versions

Celeste Classic is fully available in this game.

i did not know before this list that celeste classic came first

-Virtua Fighter 2
-Cyber Troopers Virtual-On
-Toylets (does this count...?)

-Space Harrier
-Fantasy Zone
-OutRun
-Super Hang-On
-Virtua Fighter 5: Final Showdown (holy shit)
-Puyo Puyo

A full remaster of Timesplitters 2 is playable through a cheat code button combination. The button combinations can be found in the following link:

https://www.polygon.com/22375774/timesplitters-2-homefront-the-revolution-easter-egg-cheat-code-unlock-how-to

The entire Yakuza series features different classic Sega games in most entries. These are in 0:

-Space Harrier
-Fantasy Zone
-OutRun
-Super Hang-On

Includes "Speed Mario Bros", which is identical to Super Mario Bros, but at double the speed.

Includes "Super Luigi Bros", which is just Super Mario Bros but the stages are reversed and you play as Luigi.

Includes "Luigi Bros", which is just Mario Bros where you play as Luigi by default.

The original Flashback is available from the main menu.

-Virtua Fighter 2
-Taiko no Tatsujin

The multiplayer map Nuketown 2025 has an easter egg that allows the player to play a few Atari games. Nuketown 2025 is a DLC map that only specific versions of the game could get (i.e. pre-ordering, or special editions). This is a list of the games and a video guide to the easter egg:

-Pitfall
-H.E.R.O.
-Kaboom!
-River Raid

https://youtu.be/yn0oy4tgR7M
(I apologize for the Rooster Teeth jumpscare)

Includes The Ultimate Doom and Doom II.

YMMV on if this counts, but you can toggle between the remake and an updated classic Xbox look. I think it counts.

Sonic the Hedgehog (Genesis)

Splatterhouse 1, 2, and 3 are unlockable. But no Wanpaku Graffiti? So sad.

An easter egg allows you to play through Zork I: The Great Underground Empire.

By beating the first level after shooting down four planes or by entering a cheat code, the player can play "Indiana Jones and the Fate of Atlantis", complete with Wii pointer controls.

The original Contra and Super C could be unlocked by beating various challenges.

Includes Castlevania: Rondo of Blood and Castlevania: Symphony of the Night.

Includes the original Rampage and Rampage: World Tour

Includes the original Metal Gear and Metal Gear 2: Solid Snake (with changes from the mobile versions).

Includes the original Pac-Man.

The 80's TMNT arcade game can be unlocked.

This release gets rid of the the NES trilogy, but adds the original arcade Ninja Gaiden.

Mortal Kombat II is unlocked by completing five hidden missions as Smoke.

The GameCube version includes Super Punch-Out!!

Finding all hidden drug stashes in the game will unlock the original N.A.R.C. arcade game.

Xevious can be unlocked by getting all silver medals in the game in any region. Exclusive to Japan, Battle City and Star Luster could be unlocked by getting all bronze and gold medals respectively.

The 80's TMNT arcade game can be unlocked.

Includes the arcade versions of Tekken, Tekken 2, and Tekken 3, as well as the game StarBlade.

Includes TRON and Discs of TRON.

The original Xbox version has the classic NES trilogy unlockable.

A cheat code unlocks the original Pitfall.

The original Metroid is playable after beating the game.

Includes the original OutRun.

Includes a remake of the original Mario Bros.

The original Prince of Persia can be found by finding a secret area. The GBA version can unlock the game for the GCN release by being linked and activating three switches.

I had a hard time finding information on this, additional information would be appreciated.

Features a multiplayer version of Star Wars Rogue Squadron II: Rebel Leader, three arcade games - Star Wars, The Empire Strikes Back, and Return of the Jedi - can also be unlocked. The former two by simply playing through the game, and Return of the Jedi by using the cheat code !?ATH!RD GAME?YES

Includes a remake of the original Mario Bros.

Features all of Sonic's games from the Sega Game Gear:

-Sonic the Hedgehog
-Sonic Drift
-Sonic Chaos
-Sonic the Hedgehog Spinball
-Sonic Labyrinth
-Sonic the Hedghog 2
-Dr. Robotnik's Mean Bean Machine
-Sonic the Hedgehog Triple Trouble
-Sonic Drift 2
-Tails' Skypatrol
-Sonic Blast
-Tails Adventure

Includes Wolfenstein 3D

Includes "Dr. Wario", a more Wario-themed version of Dr. Mario.

Includes the original Panzer Dragoon.

The original Metroid can be played if a completed copy of Metroid Fusion is linked with a GameCube. It is only unlocked in Prime.

Includes a remake of the original Mario Bros.

The North American version includes Virtua Fighter 10th Anniversary Edition.

Beating the Xbox version unlocks The House of the Dead 2.

These games can be unlocked:

-Pac-Man
-Ms. Pac-Man
-Pac-Mania
-Pac-Attack

Features several NES games in full. Some are unavailable without external sources, but will be listed anyway:

-Balloon Fight
-Clu Clu Land
-Donkey Kong
-Donkey Kong Jr. Math
-Excitebike
-Golf
-Pinball
-Tennis
-Wario's Woods
-Baseball
-Soccer
-Donkey Kong Jr.
-Donkey Kong 3
-Punch-Out!!
-Mario Bros
-Ice Climber
-Super Mario Bros
-The Legend of Zelda

Includes a remake of the original Mario Bros.

An updated version of Madden '92 for the Genesis is playable.

-Space Harrier
-OutRun

Includes a remake of the original Mario Bros.

The NES Excitebike is available in the "special tracks" menu.

The original Space Invaders can be found.

-Space Harrier
-Hang-On

Includes the original Donkey Kong and Rare's Jetpac

Beating the game unlocks the original Space Invaders.

Super Mario Bros: The Lost Levels can be unlocked

The original Asteroids is available straight from the main menu.

Beating the game unlocks the original Elevator Action.

The PlayStation version includes the original Galaga.

A cheat code unlocks the original Pitfall.

Includes a remake of the original Impossible Mission.

Classic Pac-Man and Ms. Pac-Man are playable in an in-game arcade.

Maniac Mansion is available on an in-game computer.