Thunder Force - Thunder Force V
I think this is the best of Technosoft can do in the series. Not because the first time the series implementing 3D aspects in most scenario, but because how few things gets reworked entirely in balance, like how CRAW was actually rechargable (or disposable if I remember correctly). The flow of the game is also goes really well, even though, I never finished it before due to difficulty. Either way it's really fun to play.
Castlevania - Castlevania: Portrait of Ruin
Even though it's not my favorite games in the series, I think it suits here because how Konami tries to make players to get used to control 2 characters at a time. The game is not too difficult, and just some puzzles here and there...
Metal Slug - Metal Slug 6
While the and the graphic is not the best the devs could do, but I think the gameplay is gets pushed to its limit, by introducing many things into the game like unique skills on each characters, and new mechanic where you can hold up to 2 weapons (and also can be thrown as well). The actual gameplay itself can be rather punishing, specially on the hard difficulty, but it gives some sort of satisfaction as well when the game is finally completed.