XT rear mech is pretty much the gold standard.
Modern BB systems are not really much of an improvement in long term reliability. I doubt if ext bearing XT will outlast a Phill Woods/TA Zephyr combo and the intermediate steps of octalink and ISIS are a convenience at the expense of bearing life.. Every bike company is really waiting for a new standard of over-sized BB shell so hollow spindles can be set in proper sized bearings inside the shell.