Manic Miner has aged well, but my pick would be Head Over Heels, and similarly Batman (isometric). Mainly, action games don't age well I think. Although, one exception would be Goldeneye. Even though it looks like yesterday's lunch nowadays, I still play it even now (Perfect Dark too, which pips Goldeneye at graphics and interface, but doesn't quite match it for level design).
Actually, Green Beret on the Spectrum too.
Actually, Green Beret on the Spectrum too.