You have correctly suspected that the answer depends on your defintion of "average."
One such definition is "a hand that wins an all-in heads-up match 50% of the time": according to that definition,
Jazbo's calculations say Q5o wins 50.12% of the time while J5s wins 49.986% of the time.
Another such definition is "a hand that wins 10% of the time in a 10-way all-in matchup", in which case 63s is in the middle, while hands as weak as 43s are better and hands as strong 98o and A6o are worse than average.
Another is to look at actual play: according to
PokerRoom's records people have won money overall at their site with A2s and 98s, and better, lost with T8s, K7s, and worse (but because their site is raked, an "average hand" might be expected to lose money, 0.03 to 0.05 big bets per deal, and you might move a few lines lower on their chart.)
The value of hands falls off very steeply at the top, and then flattens out very quickly, whatever method you use. There is a huge range of hands that are 'close to average', and you can make a case for any one of them, for some definition of average.