I vote for Robocop, but none of the version is perfect, C64 is the closest because of the soundtrack, but there's only one life, so you will not go to far if you aren't a manic gamer. Speccy version is monochrome, so it looks poorly. On Amiga the music is different (based on the movie soundtrack), not so jawdropping as the C64 composition. CPC version is very good, but all emulators I have don't allow to associate games with the emu and run them with one click, so I very rarely use CPC. I don't remember ST version.
The second place is for Chase HQ, with very good music and graphics on each platform, but the game in itself is boring.
While Cobra does have a certain charm, boosted somewhat by the hardcore-fan aspect, Head Over Heels is a far better game. It may fall foul of the dreaded isometric problems of not truly being able to tell where things are in relation to each other in ISO space, but it had charm, gameplay and that annoying "one more time" appeal to it. It beats Ritman's Batman by a country mile, good though that was. HoH is one of those games that truly does deserve CLASSIC status. A sequel is long overdue and would certainly be welcomed by the geek fraternity. But sadly today's gamers all use those horrible button-less mobile devices, so I can't see it sitting easily on any of those. If someone in the coding world out there wishes to prove me wrong, then my pixel skills are more than ready to pitch in.
I loved Robocop - i think it was the 128k music that made it for me more so than the game itself. Still have a Robocop Gameboy cartridge that I can stick in the GBA SP to listen to the music - a little slower on the Gameboy but it is still the same track.