I’d hazard a guess that most people view ROTS as having more ‘Star Wars stuff we wanted to see’ in it than the other prequels (Vader, Rise of the Empire, Anakin’s seduction to the Dark Side, Anakin and Obi-wan’s friendship), and so by proxy it becomes the best of the prequels.
I fall into the camp of liking TPM the most, simply because it is dissimilar enough from the OT to be its own thing.