If hydrostatic equilibrium IS required
As far as I know, the only effect that can make solar system objects that are in hydrostatic equilibrium deviate from a sphere is rotation. So as JamesK points out, Telesto would probably not count if hydrostatic equilibrium were required. I say "probably" because right now I can't proove that it's not in hydrostatic equillibrium - I can't find a rotational period for Telesto. But if you melted Telesto so that hydrodynamics could happen, I think it would become much more spherical.
In this case, Saturn wins.
If hydrostatic equilibrium IS NOT required
On the other hand, if hydrostatic equilibrium is not required then probably ʻOumuamua is the winner with a longest to shortest ratio somewhere in the ballpark of 6 to 8 according to that article. There are no direct measurements of its dimensions, but careful analyses of its reflected sunlight curve as it tumbles has been used to estimate its 3D shape.
That huge ratio and narrowest dimension of tens of meters to of order 100 meters evoked the "ancient & dead(?) rocket ship" meme.
What celestial body (inside the solar system)
ʻOumuamua is currently inside the solar system, but it is slightly gravitationally unbound, so it will not be coming around the inner solar system again. Instead it will leave the solar system in the far future and continue on to generate sensational headlines for other species to get excited about.