Ecco the Dolphin, via a Megadrive emulator. It's
insanely difficult and unforgiving, I think it was an act of unbridled psychopathy to inflict this on kids.
It's still a great looking game though, which is the benefit of 2D sprite-based graphics - not held back by polygon count, the sophistication of the lighting engine, or the CPU's ability to handle physics processing. The only limiting factor is resolution, and while the 320x112 offered by the Megadrive isn't great, it's still enough to render something pretty.
Feels great to play, too. They really nailed the fundamental dynamic, it's precise and physics-y in the way that made Sonic work so well. Just a shame they then had to make it so prohibitively tough. Oh well, got the much easier, prettier and generally better sequel to look forward to.