There are two for me first is Soul Star on the mega cd, never new about this one until A.J's video for My life with sega, what an amazing game, similar to starfox but in my opinion much much better.
The other one is comix zone on the mega drive, I know most sega fans know of it but the wider world missed out on this as it came at the end of the mega drives life when everyone was jumping to saturn or playstation. I introduced it to my 12 year old son and he loves it even compared to Call of duty zombie battlefield 7!!!!!