I'll rank since grades are mostly meaningless. 1) Microsoft, 2) Sony, 3) Nintendo
Microsoft's had problems here and there. It started out really boring until they brought out the star power. The Modern Warfare 2 demo was just a terrible part to show. The Forza 3 part should have shown more about the game, even though it was very clever how they addressed all concerns from Forza 2 in a short clip (cockpit, roll over, not ugly); most of that time was spent showing just how petty they could sound. And the Natal footage shown was too much of a disconnect from what was demoed live, even if what was demoed was very impressive.
Other than the few problems, you had a bunch of great games shown off. Reemergence of both Splinter Cell and Alan Wake, both looked great. Halo ODST demo with new game announcement and the announcement that the beta for the new game will ship with ODST. Crackdown 2 trailer. Metal Gear Rising on 360 with Kojima on stage. Cool looking Metroid style game for XBLA. Got two Beatles on stage. Facebook, Twitter, whatever the hell else. 1080p movie streaming. Easier Netflix usage and that virutal room thing. Ending it with Natal and showing how cool that was. I'm forgetting some stuff, but it pretty much covered all bases. The pacing was nice and didn't have long boring parts like the other two.
Sony's was really uneven. First off, stop talking about the PS2. Don't talk so damn much. Microsoft was smart to say that they will show off games, and you almost got an idea that Sony would do the same at first. Gran Tursimo Portable was like an energy suck. MAG was a poor demo choice considering the concept. A bunch of new PSP game announcements, no looks or demos at the games or unit. The PSP-2000 had more coverage in 2006. A lot of people fault the showing because things were leaked early, but what a stupid way to think; they planned this stuff and couldn't change it just because things leaked. And don't ever talk about Home.
The conference did start great with the amazing Uncharted 2 footage. Then boring shit for a while before PSP announcement. A lot of good games were shown, which is a plus, but they really needed to show off Gran Tursimo being played. Metal Gear Peace Walker trailer was great. Assassin's Creed 2 demo was really incredible. The Last Guardian trailer was the single best trailer at E3. God of War 3, another amazing demo. ModNation Racing was neat, especially because it's pushing the creating and sharing thing. Final Fantasy 14 was the biggest "wut" moment of E3; I thought he meant Versus XIII. The motion controller was neat as well. They were able to show it off right there in there, no faults, working in a variety of applications. It basically improving the parts I like about the Wii; but since it's Sony, it will most likely not surface again. Overall it was solid, just not as crazy and good as Microsoft.
Nintendo's, in the end, really came down to two trailers. Galaxy 2 is probably goty 2010 and Metroid looks interesting, but the rest of that conference was a stinker. New Super Mario Bros Wii is a good announcement, but like with Modern Warfare 2, the demo of it was so boring. I didn't even realize how much I wanted the game until after the black hole of excitement that was the crowd up there. Golden Sun DS and the Mario DK game on DSiware were also two other great announcements. But that's all there was there. They should have had more, and actually good, demos of games up there. They picked the two worst looking Wii Sports Resort games to show off. As if that wasn't bad enough, they had an extremely awkward bit with Reggie and that other guy. Luckily they had the cool archery demo at the end. The rest of the time was just boring. The only new product they revealed, but didn't show, was the thing you stick your finger in. ok.