Sonic CD, a Sonic game originally released for the Sega CD / Sega Mega CD back in 1993, was much more than just a brilliant game. It was one of the best Sonic offerings ever, a properly huge platformer and a game that really showed off what the storage space of CD-ROM could be used for: top graphics, tons of levels, lovely animated sequences and a fantastic soundtrack. It was, in a nutshell, a game that promised a new era and an oddly overlooked (compared to other Sonic games, that is) classic.
And now for the good news. Sonic CD has been ported to iOS in a most excellent version. You can grab it over at the App Store and enjoy a brilliant, innovatively designed and frantic game complete with a ton of extras, multiple soundtrack options, achievements and more. Oh, and according to this most informative Sonic CD review by Toucharcade the quality of the emulation is simply unbelievable.