Sonic CD was originally made for Sega's CD peripheral for the Mega Drive/Genesis, but it soon left that device behind. It came to PCs in the mid-1990s as part of Sega's foray into PC gaming, and has been in various collections since. But it wasn't until the 2012 remake that people took notice.
I love this game, not least because it introduced a larger story to Sonic. Metal Sonic kidnaps Amy Rose, which has Sonic racing through increasingly labyrinthine levels, culminating in a showdown between the blue hedgehog and his metal adversary in a race, before facing off against Dr Robotnik in the final boss battle. It also introduced time travel, multiple endings, and special stages where you destroy UFOs.
This was the remake that began a small renaissance for Sonic the Hedgehog. It got rid of loading times when switching between zones, included both the US and Japanese/European soundtracks from the original Sonic CD, touched up the FMVs created by Toei Animation, and even included its own touches, such as the Sonic 2 style spin-dash and Tails as an unlockable character.
All these improvements took a Sonic game that for years divided fans, and turned it into something worth playing. On top of that, it was Christian Whitehead's gateway into a working relationship with Sega, and that's important. Without Sonic CD, we may never have gotten Sonic Mania. It may not be the Sonic game for you, but nobody can take away its legacy.