Sony: Has won E3 all these years, their openings of orchestral music is legend. They have a lot to show in terms of games. Will be epic.
Microsoft: Since they are weak, they might actually have a good showing by throwing the bomb of the Xbox Next. Still they will lack in games, boring.
Nintendo: they are coming back to E3 and will show off a lot of amazing games. They will announce new Metroid, Kart and Smash. Plus, some new i.p, in addition their next gen handheld system.
My pick: Nintendo. I think both Sony and them will have a great conference. However, Nintendo has more meat to show.