Alex Kidd in the Enchanted Castle is a 1989 platform game released for the Sega Megadrive/Genesis. It was developed in-house by Sega as a competitor to the Nintendo Mario games, which were hugely popular at the time.
Released exclusively for the Sega CD in 1993, Sonic CD is arguably the best game in the entire Sonic the Hedgehog series. It came out between Sonic 2 and Sonic 3.
When Sega bigwigs asked their talented designers to create a video game character that would become the company mascot, the designers rubbed their chins for a while, then came up with a blue hedgehog, called Sonic.
In fact, the designers were so enamoured with their new creation that they changed their name to Sonic Team as they developed the game.
My personal favourite of the Alex Kidd Sega Master System games – Miracle World was first released in 1986.