first, you need to realize the difference between Raw, +, and # stats. Raw (OAV, for example) is self-explanatory, and + (OAV+) is how far above or below league average they are in that stat. however, neither will tell you everything, that's what the # (OAV#) is for. so for whatever stat you're looking at, # is the best in general.
as for the best stat, ERC is all encompassing. i always start there. it is basically a combination of WHIP and HR/9, while WHIP is a combination of BB/9 and OAV. so once i start with ERC, i then move on to BB/9, OAV, and HR/9 to fit my stadium.
1. ERC#
2. BB#, OAV#, HR#
3. K#, ERA#