that's tough. all of the choices have their merits.
if i had to choose, i would pick the big W.
all of the slam final scenarios have the potential to be classics.
the australian would provide a nice rematch of the AO09 final on a surface where they are almost nuetral, (a relatively slow/medium speed, high bouncing hardcourt). Although the AO has a highly electric atmosphere and really tough conditions, i dont find it as prestigious as the other slams, therefore i don't really like the idea of their last slam finals meeting to be in the AO.
between the other 3 choices, it was a lot tougher to choose. the french came in third in my book. it would be very nice to see an RG final between the two, let alone their last slam encounter ever(in the finals). but no matter the outcome, i dont think it changes anything. if fed beats nadal, it doesn't make him a better clay courter, perhaps a better player all-around, but not a better clay courter. if nadal wins, it just further solidifies his status as one of, if not the, best clay-courter ever.
the USO came in a close second. it is the only slam nadal hasn't reached the final of. it is the only slam nadal hasn't won. if he wins it will complete his career grand slam. it may also silence some critics who don't hold any credibility to his fast hard court game and his endurance/stamina in the latter end of the season. if fed were to win, he will arguably be the best hard court player of all time, but it can also be argued that he already was.
wimbledon is where i see the most drama unfolding. it is the most prestigious of all majors, where both fed and nadal have a fair chance of beating the other. from how i see it, and i will sound really biased here, it will allow fed to show that rafa is not as good of player on grass than he is. there.