Okay, maybe we're getting ahead of ourselves; Olympus Mons is the largest mountain we've seen evidence of on the universe so far; but it's still an impressive, monolithic structure, and it's going to take some beating.
Olympus Mons is gigantic. It's so tall, in fact, that it almost punctures the Martian atmosphere and sticks out into space. Even the comparatively primitive telescopes of the late 19th Century could make it out from Earth, and our best estimates suggest it would take anywhere between six months to a year to climb it. The slope is very gentle, and you could walk up it without feeling the effect of the rise, but a combination of low oxygen and freezing temperatures towards the summit would present their own risks. And that's before you take into account the fact that it's an active volcano. So perhaps best not!