So out of curiosity I took ESPN's RPI rankings and averaged them by division. Those rankings have issues, but they are at least objective and take into consideration WHO each team won against. We've also had enough interleague play now that they are beginning to show the difference between the AL and NL. (Note that the AL isn't as much better than the NL as in years past... the AL is currently 75-65 against the NL, which would project out to roughly 87 wins over a 162-game season.)
Here are the RPI averages by division:
* AL East - .5254
* AL Central - .4902
* AL West - .4975
* NL East - .5128
* NL Central - .4882
* NL West - .4884
So, yeah, those people that said the NL West is the weakest division in baseball were wrong.