Golvellius on the Sega Master System.
I've owned it since the 80s but back then couldn't get very far. The fact that it uses passcodes instead of saves didn't help. Last month I played with a mod which adds save support, on the MiSTer FPGA (which I have installed in a C64 shell), on a nice ~10inch OLED.
I got all the way to the final boss but... still haven't beat it.
I mean, it's one banana. What could it cost, $10m?