Because everyone loves lists, here's my personal favorites of E3:
Rock Band/ Guitar Hero: World Tour - I'm digging World Tour's song creator and its subtle additions to the "hitting circles in time" formula so I'l probaby pick that up for the whole set. Still, Harmonix has really stepped up their song selection from RB1, both in terms of quality and quantity. 84 on disc songs 20 free dlc songs and the fact that I can probably borrow the RB1 disc from a friends to get all of those is an incredible deal.
Prince of Persia - It looks like more of the same gameplay that kept me hooked during last generation's PoP trilogy, but I'm really digging the style change from gritty action to fantastic story book. It also looks like they've nailed the "no death" formula so that it's approachable yet you're not going to waltz through it without developing a strategy.
Mega Man 9 - How can anyone with a soul hate on old school Mega Man?
Resident Evil 5 - Call it cautious optimism, I like the RE4 formula, but this looks like another step away from horror and towards action. Plus, the story still sounds shitty, which doesn't work without the b-horror backdrop. I'm sure it'll be tons of fun even if it's not quite what I want it to be.
Fallout 3 - Maybe it's not everything that purists want it to be, but I personally think the Oblivionishness will play well with the Fallout world. Plus, 100+ hours of post-apocalyptic awesome is pretty awesome.