Or rather, for which of the three console manufacturers is it more important to have a good E3 showing this year?
I think few people would say Sony at this point. They can just drop the mic and walk off stage, and still have a good year.
Nintendo is going through a bit of a drought after a fantastic first year, but at least we know that Smash, Fire Emblem, Pokemon and Metroid are coming later on.
So I would probably say that it's Microsoft. And it may be one of the most important E3's for them in a very long time, outside of console launch years.
After the cancellation of Scalebound and Fable, and mixed reception towards Sea of Thieves and State of Decay 2, I believe the only big exclusives we know of coming out for Xbox are Crackdown 3 and the sequel to Ori.