The tallest mountain I've seen so far was about 9,000m high.
I was only able to assess its prominence because it was basically vertical on one side, so I could check its height immediately with my ship.
Unfortunately if mountains are part of a bigger, or wider range it becomes harder to asses their height due to the lack of a reference point (but on the other end they would not look as interesting).
Olympus Mons on Mars is very high, true, but its base is about 20x its height, so it looks more like a gentle slope than a massive mountain.