Maybe I'm wrong but to me it looked like a really in-depth live stream of games announced (there were even unannounced games I think) during the main Nintendo digital show. Sony have a similar thing on their YT channel but that's more like the official version of what GameSpot/IGN do and so not very in-depth.
Nintendo, easily. The best 'conference' (it was genuinely funny in palces and had some great material in it for fans and forums to chew on) and as said by numerous others, Treehouse put it a step above. I saw a Bloodborne Impressions video on Gametrailers that was two people sat in a room talking about the game with clips from the trailer interspersed. I'm currently watching a 45 minute video of Xenoblade Chronicles X with masses of gameplay on show. Bloodborne will almost certainly turn out great but in this scenario it's incomparable, Nintendo have had the best showing of their games (in the manner of making it easily accessible for people not at the convention to watch gameplay footage).
Also I was quite taken aback by how multiplayer-focused their revealed titles were. Bayonetta, Yoshi, Hyrule Warriors and Splatoon all confirmed multiplayer games (I think those 4 were online online confirmed as well, right?) and then there's the looming beast of Smash Brothers over the horizon which threatens to consume my life.
I liked some stuff from the other conferences but it was all very prescribed and predictable, with few genuine surprises. Admittedly Nintendo were much the same, I guess it comes down to that notion that I'm just more interested in what they're producing than Sony or Microsoft. I have never enjoyed Halo, LittleBigPlanet does nothing for me etc.
Nintendo. Exciting content and Zelda looks stunning.
Microsoft were a fairly close second, they had the one exclusive that could entice me to a next gen console in Sunset Overdrive and the announcement of a new Crackdown was also a highlight. Would have been nice to see Quantum Break or a new Banjo game.
Sony did little for me. Remakes of games a year old and a new Uncharted did nothing for me. The Order is the only game that really interests me, but there was very little that interested me sadly.
I'm honestly starting to question my sanity. I thought nintendo's E3 stream was awful but everyone loved it... I mean I thought it was dreadful. I want to play Sony and Xbox games far more...
I thought all 3 platform holders had great conferences and for once concentrated on games for gamers and not casual gooseberry fool or motion controlled bollocks. Maybe Nintendo edged it but the only downside from this years E3 was so many games being pushed into 2015.