Daiku no Gen-san - Robot Teikoku no Yabou is a Gameboy platformer with some very impressive graphics and a variety of gameplay mechanics.
This game has much more varied gameplay compared to the first one in the series, but the overall optimization task is much simpler. There aren't a lot of options in most cases, and the biggest challenge comes down to lag reduction. The single most important time save is freezing the penultimate boss in place with frame perfect attacks.
Gameplay aside, the Gen-san series is also technically complicated, requiring new tests of mode 3 timing to get console verified. This particular game led to many emulation improvements in GBHawk, and this run is one of only 2 I know of that will desync in Gambatte.
Here is a console verificaion video:
feos: Judging...
feos: Removed a blank frame at the end of the movie.
feos: Gameplay looks good, but feedback is too mixed to sent it to Moons. Accepting to Vault.