I had a portable pirate Sega Genesis as a kid which might not have had enough cartridges to use in it, and without built-in games, my selection was of 8 titles. My brother also had a portable pirate Sega Genesis, more closely looking like a Nomad, he had tons of games for it but the sound quality was pretty off.

This right here was my favourite to play from the 8 titles I had for the little system, and the music was the first thing that came to mind while replaying this as an adult. The other thing was that it was pretty short, but refreshingly so, at only 4 gigantic levels that you had to fully explore to complete the missions, involving a memorable central set piece to each one of them (especially coming from the SMS version, the spaceship you have to climb through the whole level feels properly huge). Of course it's nothing groundbreaking, and it draws a lot of inspiration from one of my least favourites Bond movies (Moonraker) making the settings have a lot of sci fi elements to them, but it's solid as far as retro games go.