Just my humble opinion:
1. Fallout 3
2. Metal Gear Solid 4
3. Dead Space
4. Star Wars: The Force Unleashed
5. Gears Of War 2
6. Rainbow Six Vegas 2
7. Grand Theft Auto 4
8. Battlefield: Bad Company
9. Tomb Raider: Underworld
10. FIFA 09
Any of my Top 3 could have taken the number 1 spot, as all 3 were outstanding.
Fallout 3 is one of the most atmospheric games I've ever played & I've chalked up 100+ hours on it.
MGS4 - Best graphics I've ever scene on a console & despite the daft-in-places story & the boss battles not being as strong as MGS3, it is still a worthy finale to the story of Solid Snake
Dead Space was the surprise package for me in 2008. Normally when you see the EA logo on a non-sports game you stay well clear, but the great visuals, tense atmosphere & solid gameplay make it a serious contender to Resident Evil & Silent Hill in the survival-horror genre. It's THAT good.
Star Wars: TFU also surprised me. After reading all the negative reviews, I was pleasantly surprised (& relieved) that most of the reviewers got it wrong. The story was exceptional & involving: probably the best story in any game in 2008. Yes the targetting system had it's flaws, but it didn't ruin the downright fun gameplay (Using the Force to pick up Stormtroopers & impale them with your lightsaber in mid-air never gets old.
Gears 2 & Rainbow Six Vegas 2 were decent sequels, but weren't much different to the originals.
GTA4 felt like a bit of a step back for the series. I felt Rockstar took too much out of the game. There were not as many weapons, no planes, couldn't customise your cars. Also the missions were pretty much all the same: Drive here, kill this guy, escape from police. The missions didn't have the variety that San Andreas & Vice City had. However Liberty City looked tremendous & it was great just cruising around, bumping in to random people, or going on a killing spree to see how long you could last.
Battlfield: Bad Company was one of the better military FPS shooters I've played. Far longer & FAR BETTER than the woefully overrated Call of Duty games.
Tomb Raider Underworld is the best-looking Tomb Raider so far. The levels were well done, but they were a bit too easy to get through (compared to the earlier games) With a bit of tweaking, a bit more thought into the puzzles & removal of the stupid storyline, this could have been the best Tomb Raider of the series. It is still good though & worth a play through.
FIFA 09 - The last FIFA I bought was World Cup 98 for the PS1. Then I switched to the Pro Evo football games. But since Pro Evo 3, the games have been getting worse with each release & Pro Evo 6 was abysmal. I'd all but given up on football games until my Xbox 360 broke & I had to get a new one that came with FIFA 09. It plays just like a more refined version of the old Pro Evo games, but with better graphics & sound AND the official FIFA license with proper team & player names. A welcome comeback for the FIFA games.
Fable 2 & Prince of Persia could also have made my Top 10...... but I haven't got round to playing them yet !!! Both are in my collection. Fable 2 is still sealed & I'm just about to start Prince Of Persia. So much to play, so little time......