Which is better? = Ποιο είναι καλύτερο; without το.
As Dimitra points out in her comment Ποιο είναι το καλύτερο; could also mean Which is the better one of the two/ among the group?
It could mean the best out of two things, or the best out of all, or practically any number of objects that the speaker is referring to. It depends on the context ^.^