Yes, roundest is an adjective. However, it may not always be appropriate, depending on the context.
comparative - rounder superlative - roundest
The roundest object in the world is a silicon sphere created by the International Avogadro Project. It is considered the roundest object because its diameter is consistent to within a few atoms, making it incredibly smooth and symmetrical.
The roundest object in the universe is the electron. It is considered the roundest because it has a perfectly spherical shape, with no detectable deviation from being perfectly round. This makes it the most symmetrical and uniform object known to science.
20
1300
It is: 254.3
9000
5000
100
8910
5
It is 0.188