BIG NO to GTA IV on PC. Massive failure of a port.
The best singleplayer game of the last 5 years from what I've played is Bioshock. The game takes place in a dystopian colony at the bottom of the sea. It is set in a sort of high-tech 1930s world, a genre sometimes referred to as "steampunk" and is something of an homage to the Ayn Rand novel, Atlas Shrugged. Developed by 2K Games, in large parts by their studio in Canberra. A sequel is forthcoming this year.
Empire: Total War is the latest and most epic instalment in Creative Assembly's long running strategy series. The gameplay is a combination of turn based and real time strategy, set during The Age of Enlightenment. Manage your empire, or champion a revolution. Fight it out on land or at sea in real-time battles.
I'd be silly not to mention the Orange Box. Portal, Team Fortress 2 and Half-Life 2 with Episodes 1 & 2. A bunch of superb games for the price of one.
warning: high level cartoon violence
Call of Duty 4 is still being played online and another game that has a sequel due out later. It's a pretty straightforward combat FPS and that's why it's popular.
Fallout 3 was quite a hit too, a first person "action role playing game" set in a post-apocalyptic world. A number of downloadable packages have been released for it, to favourable reviews.
In terms of indie/puzzler/casual games, try
World of Goo
Braid
Audiosurf
Peggle
Plants vs Zombies
Blueberry Garden
There is also a new option, simply play a game for free online. We're not talking about little flash arcades, either.
QUAKE LIVE is a reboot of Quake 3 and Team Arena. Download a browser plugin and install the game, then all you need to do is login to the website when you want some action. It's a fast game and will run on pretty much any system, while the in-built matchmaker aims to give you a game that matches your skill.
Battlefield Heroes is a simplified version of the Battlefield series. Create a character in one of three classes and join battle in a cartoony WWII environment, complete with tanks, aeroplanes and jeeps. As your character earns experience, you can spend points to gain new weapons and abilities. You can also spend real money to enhance your characters quicker and play dress ups.