Just like everyone else, it's definitely "World of Warcraft: What's Next" and/or "World of Warcraft; The Adventure Continues." Which ever one of those previews the next expansion.
Diablo III: Reaper of Souls Preview, Im a huge Diablo fan and i am as excited as ever to see what exactly is in store for the game. Also the Wow whats Next is a big reason for me to be paying close attention due to the fact that iv been playing since vanilla and hope we can finally defeat Sargeras( Fingers crossed :P). I'm most excited for those 2 panels especially but in all id probably watch it like 3 or 4 times if i buy the virtual ticket because its something me and my friends look forward too every year!
What I'm looking forwards the most to is... the Opening Ceremony, World of Warcraft: The Adventure Continues, Art of World of Warcraft and the Closing Ceremony.
Heroes of the Storm Overview! After seeing blizzard dota at blizzcon a few years ago, I am excited to see whats changed and I have recently gotten into moba games, so I want to see how blizzards game will hold up. I am sure it will be amazing.