i think ~average is best maybe bit below or above depending... many factors involved in results, so hard to distinguish.
alot of this depends on how things are defined:
flyballs are the easiest outs by their slash. (somehwere like Fenway may not apply)
line drives have the highest slugging/best slash (probably similar to a "hard hit ball rate" you hear about, some of these may get some altitude but are not 'flyballs')
groundballs have a higher average than flyballs, but fewer XBH likely.
Then, how does this all translate to OotP, specifically -- even more grey added to the picture
defense is going to be key for groundball pitchers... not as important for a fly-ball pitcher (if you read that as not important, not my fault)... etc etc... no 1 answer. it's all relative to the forces at play at that time.
so, what might be best for regular season may not be best for the playoffs... what's best for a leadoff hitter might not be what's best for a power hitter... since we can't swap them out in an unlimited way, you gotta weigh things and make a good choice.