I am biased toward thinking Justice Scalia is the best legal mind on the court, but legal commentators (on the left and right) have described Justice Thomas as the best legal writer (i.e., clear with tight reasoning).
Given her past record, Ms. Sotomayor will likely be the worst writer on the court in many years, unless she picks unusually talented clerks.
A lot of court-watchers seem to be swayed by how active a particular Justice is in asking questions of the attorneys. Thomas is usually pretty quiet from what I’ve read.