The COD games on Wii. To have the full Modern Warfare campaign on the little system is still astonishing.
Resident Evil 2 for the N64. While power wasn't an issue, the limited cartridge space sure was. This was exactly the sort of game that was supposed to be impossible on a cartridge.
Much of the Dreamcast library, from Crazy Taxi to Marvel vs Capcom 2. I don't think the console industry has ever seen such perfect ports of arcade games before or since. While that was basically the benchmark for the console, I think it stood out at the time as something which should have been 'impossible' for a console.