Asklemmy

What a great question.

A few that come to mind. (In no particular order)

• Socrates and Aristotle - literally changed the way humanity as a whole thinks, their influence echoes thorough time their ideas are talked about 2500 years later.
• Marcus Aurelius - stoic philosopher and emperor, set the standard for what a great ruler could be, modern politicians have nothing on him.
• Albert Einstein - literally changed the way we look at the universe, overturned Newton and his theories have stood up to ever more precise tests.
• Linus Torvolds - the impact of the Linux kernel on computing cannot be overstated, from the smallest single board computers to the biggest most powerful supercomputers, all run Linux. The internet wouldn't be the same without it.
• Johannes Guttenburg - inventer of the printing press, the wide dissemination of knowledge that directly stemmed from this invention changed the world.
• Alexander Fleming - discovered penicillin, finally gave humanity an effective weapon against bacteria, penicillin and it's derivatives have saved billions of lives.
• Stanislav Petrov - Ignored the nuclear early warning system, which erroneously reported 6 launches from the US against the USSR. If he had followed protocol the resulting nuclear response would have triggered WWIII, he saved billions of people by not launching.

To those saying sports personalities, I doubt people will be talking about their achievements in couple of hundred years.

Edit: added Stanislav Petrov.