Jetstream Rider
Blind lines etc are not too bad and generally it is a very happy fleet. I would say the best in the airline at the moment.
I would suggest that the 747-400 is still the best fleet by a country mile, but that the 757/767 fleet is a contented fleet with a nice mix of work, and probably 2nd to the 747-400 in that it doesn't have the high number of tiring 3-day Atlantic work that the 777 has.
However, things can change, and with more 767's being dusked to LH config, the Atlantic workload will probably increase. The 777 is very capable and its workload will probably improve, but nothing will touch the 747-400 for the foreseeable future.