Gran Turismo 5 is the only must buy. However I'm watching a lot of games, be it on Wii, PS3, PSP or (3)DS, but I want some critical feedback first. GT doesn't need it, it's already perfect.

Oh man, too many to name, but I'll try because I'm bored. These are pretty much in order of my hype

Wii:
Donkey Kong Country Returns
Goldeneye Remake
Zelda: Skyward Swords
Epic Mickey
Sonic Colors
Kirby: Epic Yarn (not quite 100% on this, but the more I see the more I like)
The Conduit 2 (although now that Goldeneye is coming I may rethink that)

360:
Gears of War 3
Dead Rising 2
Halo: Reach
Hunted: The Demon's Forge

PS3:
Little Big Planet 2
The Last Guardian (maybe)
Twisted Metal (maybe)

DS/3DS:

Dragon Quest XI
Golden Sun: Dark Dawn
Pilot Wings Resort
Mario Kart
Zelda: OOT
Kid Icarus: Uprising
Paper Mario

probably more but I'm waiting until I hear/see more for the other 3DS games.

For me I'm only going to list games SHOWN at E3 2010, so The Last Guardian, Xenoblade, The Last Story, etc are omitted

Wii: 

Donkey Kong Country Returns

Zelda (of course)

Epic Mickey

Metroid Other M

3DS: 

Metal Gear Solid 3D

Star Fox 64 3D

Paper Mario 3D

Zelda OoT 3D

Super Street Fighter IV 3D

Pilotwings Resort

Kingdom Hearts 3D

DS: 

Golden Sun 3

PS3: 

Portal 2

Move set up (not sure which games yet though)

360: 

Halo Reach

Gran Turismo 5, Twisted Metal, Halo Reach, Gears of War 3, Infamous 2, and Killzone 3.

For Sure: Kirby's Epic Yarn, Metroid Other M, Wii Party

Probably: 3DS (and various games - Pilotwings!, Mario Kart, etc.), Conduit 2 (unless Goldeneye surprises), NBA Jam

Possibly: Donkey Kong Country Returns, Flingsmash, Mario Sports, EA Sport Active 2, Dragon Age dl content, Goldeneye (if it proves better than Conduit 2)

360:  Gears of War 3   maybe Vanquish

DS:  Golden Sun Dark Dawn, Okamiden

3DS: MGS 3D, Starfox, Kid Icarus   Maybe Resident Evil & Final Fantasy depending on what they are

Wii:  Other M, Zelda, Conduit 2, Goldeneye, DKC Returns maybe Epic Mickey and Sonic Colors

Rage, Fallout 3: Las Vegas, and Dead Space 2 for sure!!!

Epic Mickey and Killzone 3 are maybes.

Not enough RPGs this year =(

xbox 360: El Shaddai: Ascension of the Metatron(probably biggest positive surprise on E3), Enslaved, Vanquish, Gears of War 3, Majin and the Forsaken Kingdom(another surprise), Warriors: Legends of Troy, de Blob: The Underground(damn this game looks so fun), Need for Speed Hot Pursuit, Bulletstorm, Dead Rising 2, Mafia 2

those are my musthaves

3DS - day one buy for me.

after watching Fear 3 and Hunted gameplays and info i completely lost interest in those games and i was HUGE Fear fan.

Civ 5, most of the first half of the games on the Nintendo Channel and I think that's about it. I am tired of the mediocre shooters that have been coming out for eyars now so that takes anything MS has out the door as well as half of what Sony showed. SO I will probably nab GT5 and Twisted Metal.

Wii; Metroid Other M, The Legend of Zelda: Skyward Sword, Donkey Kong Country Returns, Epic Mickey, Goldeneye,The Conduit 2, Call of Duty Black Ops, Star Wars: The Force Unleashed 2, Lego Harry Potter 1-4,Lego Star Wars 3: Clone Wars, Ghost Recon: Future Soldier(could go PS3 if Move-supported)

PS3: Assassins Creed Brotherhood, Infamous 2, Castlevania: Lords of Shadow, Metal Gear Solid Rising, Silent Hill, Batman Arkham Asylim 2(did that even show up at E3?), The Last Guardian(even though it did not show up at E3), Socom 4(maybe), Twisted metal(maybe)

DS: Golden Sun. Okamiden

3DS: Kid Icarus: Uprising, Resident Evil Revelations, Super Street Fighter 4, Metal Gear Solid Snake Eater, Pilotwings Resor, Mario Kart