Well, it's hard to pick just 1, really. I enjoyed games from all the big 3, and I really liked EA and Ubi's conferences too. But, I'll have to go with 'Third-parties'. MS, Sony and Nintendo all showed a lot of great games that I want to play, which has pretty much cemented the fact I will be getting all 3 consoles eventually, I was picking out a few games from each conference I just needed to play, but, nothing they showed really beat either Witcher 3, Watch Dogs, Day Z or Titanfall, and a host of other third-party games too. So, I'm going to have to give it to the third-party devs.
But it was an amazing E3 all-around! Good job to everybody! I walked away from basically every conference and announcement with a huge smile on my face.