First priority... Fire Emblem - Goddess of Dawn.

2. Metroid Prime 3 - Corruption. I'm interested in how this game has evolved from its predecessors.

3. Halo 3. For the story and for the online multiplayer...

4. Metal Gear Solid 4 - Guns of the Patriots. Stealth. I never get tired of the suspenseful feeling when hiding from your foes, and then going in for the kill.

5. Mass Effect. It looks great, and being a western RPG, I'm curious to see how this game will turn out.

6. Super Mario Galaxy. Should be just as creative and enjoyable, if not better, than its predecessors.

7. Assassin's Creed. Assassination at its finest from what I've seen.

8. Resident Evil - Umbrella Chronicles. For the actual storyline and to see the new Raccoon City.

9. Legend of Zelda - Phantom Hourglass. Puzzles are fun.

10. Super Smash Bros. Brawl. Looks great, and most likely will play great.

If you like puzzle games and love to challenge others intellectually, you might want to check out Suujin Taisen, a Nintendo's new puzzle game.

This is not a very flashy fancy title, but belongs to any list of great puzzle game.  This game is based on very simple rules, yet manages to be one of the most addictive, deep, and exciting puzzle games I've ever played.  

Game outcome is not entirely determined by logic. It also involves luck. So sometimes, newbies can beat expert players, making gameplay quite thrilling.  Game items are designed very effectively, so they make gameplay more dynamic.  Each session won't take too long, so you can play it even if you don't have much time (lunch time, commute, etc).    

It has a wi-fi mode and you can compete with other people, which is simply tons of fun and keeps its replayability quite high.  Through its online play, you'll discover numerous tricks expert players use.  You can do 1 vs 1, or 2 vs 2 to do co-op.  Matchmaking is based on your rank so you won't get hugely frustrated even if you're a newbie. 

Not sure when it'll be released in the US but you might want to definitely try when it gets here: http://www.nintendo.co.jp/ds/afhj/index.html

If I buy a Wii:
Mario Galaxy
SSBB
Metroid Prime 3
RE4
Super Paper Mario

DS:

!!!!!!CONTRA 4!!!!!!
Geometry Wars
The Legend of Zelda: Phantom Hourglass

Xbox 360:
Mass Effect
Halo 3
Too Human
Bioshock
Timeshift
Half-Life Orange Box
Alone in the Dark
Assassins Creed