What is your favourite ever game on a SEGA system?

My favourite is not only my favourite SEGA game but is is probably my favourite game of all time.

Unfortunately I haven't played it for over 10 years and will probably never again.

It was a game on the SEGA master system that was built into the console.

SO WHAT IS YOUR FAVOURITE EVER SEGA GAME?

DID YOU EVER PLAY ALEX KIDD?