I'd second that mxbrady. Due to the diversity, albums that people view as strongest/weakest are always going to vary when it comes to Thrice.
For me personally, the strongest albums are the ones where the punk rock roots show through - both Illusion of Safety and Artist in the Ambulance are pretty much unbeatable for me. Identity Crisis sits isn't quite as strong as these two, but has some solid tracks (Ultra Blue, T&C, Phoenix Ignition, etc).
The weakest album for me is Beggars, purely because it was the one I felt I enjoyed the least - that being said, its still a good album.