Im gonna go with SF on this one.
He is the guy that can be the most versitile. He could be a good rebounder, good outsider shooting, good driver, good passer, good inside shooter, and I believe as a general rule, whatever offence you play tends to maximise his strengths.
If you play an inside offence, and he has good inside stats, he becomes a third inside player. If you play an inside offence and his strengths are outside, he becomes the third outside shooting player. Or again, if he has awesome passing, he becomes the link man into your PF/C.
Same could be said for outside offences.
You could have guards that are great at shooting, but if you play Low post, you arent neccassarily maximising their strengths. But a SF strengths will be maximised across the majority of offenses.
Plus he is hardest player to guard. PF's and C's you need really good inside D. guards, good outside D... SF you need both, because in your league, against different opponents, you will face both Outside playing and Inside playing SF's.