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.
My personal favourite of the Alex Kidd Sega Master System games – Miracle World was first released in 1986.
For me: THE stand-out title on the Sega Master System.
Phantasy Star is a solid – if rather basic – level-grinding RPG/adventure, and it went on to spawn a whole series of other games afterwards.