Gaiares (meaning literally Gaia Rescue, as in rescuing the Earth) is probably my favourite shmup of all time, let alone on the Mega Drive. While it was only released in Japan and the US, the US version is fully playable on a European system so importing it is no problem. The graphics are pretty awesome in my opinion and easily some of the best on the system, the music is also equally good with really good melodies and riffs that sticks in your mind for a good time after playing the game, especially the level 1 theme which I always hum along to when playing the game. The game has a pretty fair difficulty as well in comparison with a lot of other shmups from the time and seems to strike a really good balance as the game never feels unfair but rather relies on repetition to get good at it.
I pretty much learned about this game from Mark of Classic Game Room and I can see why he's such a fan of the game. It's an awesome shooter and like I said, one of my favourites of all time.