Operation C by Konami for GameBoy, released in 1991.
The early GameBoy years were bizarrely interesting. As the novelty of "a portable game system" wore off, the games developed for it needed to have some extra "oomph" built into them to merit attention; otherwise, the lower tech just made it hard to be terribly excited about these games which, while decent, were getting harder to love when compared to what was happening on the console systems (especially as 16-bit machines were captivating gamers' attentions).
And so, in this light portable Contra came into being. Konami had proven itself to be a worthy developer on the platform, and when a mobile iteration of one of their most beloved franchises was announced, it was worth noting. The game looked nice; though it was strictly a black-and-white affair, the environments looked as captivating as any other episode of the series to date.
Rather than being a re-hash of it's big brother (which could have been easily produced and would have been "acceptable,") OC was a whole new adventure with a few twists and turns to deviate from what was expected. The game was expertly assembled by a crew who knew the Contra territory quite well, and so the game played just as expected - and it felt as fresh as a "genuine sequel" would.
There's a lot of praise out there for OC, but it really did commit a really heinous act which will prevent me from praising it; this was a single-player adventure. There's certainly many reasons why they decided to design the game this way, and technically it makes sense. But this is Contraaaaaaaa (insert your "300" jokes here!) the whole point of this series to grab a buddy and get blasting! Were we not so spoiled by the first 2 NES installments which featured a 2-player simultaneous mode, then it would not have seemed to be so glaring an omission.
As it is, Operation C is a fine game, and a perfect example of the high-caliber product released by Konami during the early GameBoy years. This is a fun outing, and certainly deserving of it's place in the Contra lineage; but it definitely missed out on being something greater, due to being stripped of it's most important feature.