Want to get an idea of what ratings are the most important?
Take a look at regular season stats extended for the RC/27 stat for hitters by position, see who the leaders are, and what ratings they have. I have found it's hard to overcome poor vL and vR ratings and be a category leader.
The big league averages for fielding are helpful, but some positions affect runs allowed more than others. Fielding is tricky to evaluate because the fielding stats are distorted by other factors like pitching staff gb/fb ratios and strikeouts. There is a big thread on 'The Rightfielder' and that provides some analysis on fielding. Here is the positive impact defensive formula by MikeT23: [(('+' + PO + A) - (E+'-'))/INN]*1458.
PPPS averages for OF
LF-318.3, high end comes from range and glove, arm doesn't matter.
CF-398.8, high end comes from high range and good glove.
RF-231.4, high end comes from 65/65 arm guys.
Each out given up means you should get at least another 2/3 run of offense out of a player, otherwise stick with the better glove.
Also, when evaluating defensive performance, know that errors are attributed to glove/arm accuracy, while +/- plays are attributed to range/arm strength according to themergerguy.
While I think it's okay to care less about defense for outfield spots, it's kind of critical to have a good one a SS where the PPPS are more like 800-1000.
Pitching ratings can be a little harder to pin down, but looking at arms with high SO totals and low WHIP and getting an idea of those ratings help as well. It seems that pitching has more variables involved with control, vL/vR, velocity, gb/fb, and pitch quality.