If you mean singular objects instead of star clusters (galaxies) or galaxy clusters (superclusters), then I'll provide my list (that we know of so far):
Largest non-spherical solid object: Haumea - radius 620 km.
Largest moon: Ganymede - 2634 km radius.
The largest rocky planet: Kepler 10 c - 2.35 Earth radii (15,000 km).
The largest gas giant planet: HD 100546 b - 6 Jupiter radii (419,466 km).
Largest ring system: J 1407 b - radius 0.6 AU (90 million km).
Largest star: UY Scuti – 1708 Solar radii (1.19 billion km).
Largest black hole: radius TON 618 - 1300 AU (194.5 billion km).
Largest nebula (mostly intact): LAB-1 – 300,000 light-years across.
As mentioned above, as a bonus:
Largest galaxy: IC 1101 – 3.92 million light-years in diameter.
Largest structure: Hercules-Corona Borealis Great Wall - 10 billion light-years in diameter.
Stephenson 2-18 is now considered among the largest known stars, if not the largest, with a radius of 2,158 solar radii. UY Scuti, on the other hand, is about 1,700 times larger than the radius of the sun. It is also among the brightest cool supergiant stars, with a luminosity of 440,000 solar luminosities.