Our favorite cello string set of a single brand is Evah Pirazzi Gold/Regular.
However, we highly recommend mixing multiple string brands to get the best sound since different string brands bring out different quality sound from each string on a cello. Our favorite cello string combination is a Larsen A, Larsen D, Spirocore Tungsten G, and Spirocore Tungsten C.
You can read more about our favorite cello string brands in our cello string recommendations article.