Sonic the Hedgehog (1991)

Method 1: Original Hardware

Consoles

Android: This is a official port of Sonic released in 2013. You can choose to either play the game for free with ads or pay $1.99 for no ads. Go to Google Play Store?
Nintendo Switch: Released under the Sega Ages Line, you can get the original Sonic the Hedgehog for $7.99. Additionally, the game is backwards compatible with the Switch 2. Go to Nintendo's Online store?
Sega Genesis: The original console that Sonic 2 was released on.
Sonic Generations: If you own a Xbox 360 or Playstation 3 version of Sonic Generations, you can unlock the ability to play the original Sonic the Hedgehog within the game itself.
PlaySEGA: A PC version of the game playable in browsers with flash enabled was available from 2008 to 2011. Unfortunately, it does not seem as though this version of the game is backed up on the Internet Archive as of this current moment.

Semi-Available Methods

Sonic 2 was released onto Steam back in the early 2000s. However, with the release of Sonic Origins, you can no longer officially buy the game off of the Steam store. You can still plpay the game though if you had purchased it prior to it's store removal.

Game Compilations

Steam
Xbox (in general)

Sonic Origins (and Sonic Origins Plus): This is the most accessible game compilation to play this game on. Sonic Origins is available on Steam, the Epic Games store, Xbox One and Series X, Playstation 4 and 5, and the Nintendo Switch.
Sonic Jam: This game compilation was released on the Sega Saturn.
Sonic Mega Collection: This game compilation was released for the Gamecube and is backwards compatible with the Wii.
Sonic Mega Collection Plus: This game compilations was released for the PC, Playstation 2 (and is backwards compatible with the Playstation 3) and the original Xbox (and again, is compatible with the Xbox 360)
Sega Genesis Collection: This game compilatation was released for the Playstation 2 and is backwards compatible with the Playstation 3.
Sonic Classic Collection: This game compilation was released for the Nintendo DS and is backwards compatible with the Nintendo 3ds.
Sonic Gems Collection: This game compilation was released for the Gamecube and Playstation 2. It is additionally also backwards compatible on the Wii and the Playstation 3.

Method 2: Dolphin Emulator

This serves as another method to play the game. All you need is just a Gamecube copy of either the Sonic Mega Collection or the Sonic Gems Collection and a little determination and you'll be able to play this game on your computer. If you want to use Dolphin Emulator to play this game, go here to learn how to set up Dolphin Emulator.