My only request is that this be about ORIGINAL Game Boy games, not Color or Advance titles.
By port I mean a conversion from an arcade original or other console to the Game Boy. However, feel free to mention games in a series which converted well over to the Game Boy if you feel so inclined (Castlevania, Mario, Metroid all come to mind) if that's what you'd like to add.
My votes thus far:
Operation C (Super C port/upgrade). Much better than the arcade game; level structure is very interesting in this one. Aside from the kickass gameplay, this game sports THREE variants of the Konami Code (too bad you can't use the 10 lives code before the level select, heh) and level one is totally redone with the classic Contra level one tune. I don't feel the top-down alien hive stage fares quite as well as on the NES, but all the same it's greatly enjoyable stuff. Even the bullet flicker on pause is included! Bug: In the first top-down stage, if you walk to the top of the screen while fighting the APC which attacks from below the screen won't scroll again unless you walk back a little bit.
Ninja Spirit (Irem classic). There's a few points where enemies can be hard to see (crouching gunmen in particular), but the glorious float-and-slash gameplay of the arcade game is intact, right down to the ability to have not one shadow (Ninja Gaiden style), but TWO. True to the arcade game, again, they float. You can still stab downwards through the first level's planks with your sword. The gigantic stage one boss is here, redrawn to (barely) fit the Game Boy's screen. Sadly, as far as I know this release is exclusive to Japan.
Honorable mention: Ninja Gaiden Shadow. I don't really like the Ninja Gaiden badge on there as the signs point towards this really being a Shadow of the Ninja title, but that doesn't make the game any less fun. A Ninja Gaiden game for the rest of us. Seeing how this doesn't seem to be a real port of any game (though it has some similarities to Shadow of the Ninja for NES, the levels are all different), I can't really include it in the list of ports.
Not so honorable mention: R-Type DX (and B&W predecessors R-Type and R-Type II, included on the cart). For me, the original R-Type isn't much to get hyped over, and while this is a nice, solid port with colors that will also play on an original Game Boy, I barely manage to remember I own it. Not a bad attempt at all - just not terribly exciting.