One of the very best was Thunder Force 3 (released in 1990), this game has everything you could ask of a 2d scrolling shooter.
The graphics are superb, very detailed with fast moving sprites and very colourful backgrounds. The music and sound effect are also excellent but best of all is the level design. There are 5 stages in total and the game lets you choose in which order you play them (they include underwater, jungle and snow). Once you have completed all 5 stages the game open’s up a 6th and final stage.
Any negatives? Well the difficulty level is spot on for me ( I found the earlier games in the series way too hard) but many find this game too easy. That aside this game is almost perfect and I would have no worries in recommending it to any Megadrive owners, even if they are not big fans of the genre.