RP Mega Diancie
CM Rain Dance Manaphy
Mega Pidgeot
I voted for these three sets because they are the most diverse in terms of the type of teams we could build with them; by that I mean that they don't automatically team-build themselves. Things like RP Landorus, CM Clefable, SD Mega Gallade, SD Mega Scizor, SD Lucario etc are already well understood in terms of how they function and the kind of support they need to succeed, and therefore building a team around them would be a waste of time as it is unlikely to be much of a useful learning experience.