The Legendary Pokemon and the Legends of Isshu were amazing.
My favourite after Hoenn/Kanto.
The concept of Ying-Yang was really interesting - Zekrom and Reshiram, esp the latter have amazing designs.
They were really hyped up compared to Dialga and Palkia - I can see why ;p
Musketeer Trio! Oh MY to the EXTREME!
My favourite trio, comes close to the Legendary birds for me.
Designs and concepts were out of this world.
They're strong, elegant and beautiful.
Virizion and Keldeo are my favourite of the 4.
Victini, the victory pokemon? That's a pretty neat concept, and it's pretty cute too :]
The Kami trio are pretty interesting too, can't wait too see there new formes!
Genesect is Genesect, haha xD
Meloetta looks like another interesting Poke - most certainly will check it out upon release.
But Cover Legendaries, Musketeer Trio, Victini and Kyurem are the ones for me - Can't get any better :]