For me...
1. Mario & Sonic at the Olympic Games - A historic crossover, three fantastic licences (Olympics, Mario, Sonic), tons of great characters, numerous events to choose from (over 20 according to Gamespot)
2. Super Smash Brothers Brawl - Looks epic and awesome from the trailers and the always-nifty daily updates. Could be the best fighting game of this generation, IMO.
3. Super Mario Galaxy - Hype really went up for me when I saw how much more Mario64-esque it was and less Sunshine-like. Could be a million-seller.
4. MySims - The depth of the creativity and customization features is just incredible. Couple that with a much more fun and cute look than regular Sims!
5. Madden NFL '08 - Online play, 22 Mini-Games, Mii features, telestrator plus all of the modes and awesome motion controls from '07 PLUS improved graphics.
6. Lego Star Wars: The Complete Saga - I've played but do not own either game, and this looks awesome with motion controls, new levels and features plus improved graphics.
7. Rockstar Presents Table Tennis (2007?) - The 360 game looked awesome, and Table Tennis was always fun to watch in the Olympics. Should rock with motion controls.
8. Thrillville: Off the Rails - Always loved the Roller Coaster Tycoon games on PC and have waited for one for a Nintendo console. Not quite the same, naturally, but the new features and mini-games should really propel it to coolness.
Honorable Mentions:
Carnival Games (I know--but it has skeeball and I'm a sucker for those "dreaded" mini-game collections)
FIFA '08 (Soccer and Online Play!)
My Word Coach (My whole family loves word games and this should be interesting)
Namco Museum Remix (I've had an eye for this collection since it was announced)
NBA Live 08 (Online play and Basketball!)
Power Rangers: Super Legends (Has the ability to play as classic MMPR Rangers)
Smartypants (2007?) (A trivia game--cool)