Plain and simply, one of the greatest games of all time.
While the game's Mature rating is right on the spot, it should be noted that this isn't a murder simulator (like the idiot Jack Thompson would have you think) nor is the violence handled in a poor fashion. Anyone who thinks that this game is just an escape for immature teenagers is sorely mistaken. There's so much more to the game than just raising hell in Liberty City just for the sake of it; there's a tremendously well written main story that gives you a motivation to accomplish some of the game's wackiest missions. Sure you can go on a killing spree at any time, but while the violence is very graphic, the game handles everything in a quirky way that makes it perfectly clear it's only ok to act like a lunatic in the game, not in real life.
The gameplay in GTA III is the main attraction as there's pretty much a little bit of everything packed into the game; you'll have a plethora of different cars at your disposal, plenty of weapons, you'll perform several missions on foot that will require some exploration, you'll race around the city, hell, you can even fly across Liberty City in the Dodo (a plane that is complicated to handle and very difficult to master). Pretty much anyone who plays video games will find at least something that fits their taste in GTA III. Add to that the fact the game is very open ended; there's never one right way to do things, and you can always choose to go nuts however it pleases you. This adds a tremendous amount of replay value, to a game that's already worth every penny you pay for it.
Graphicwise GTA III gets some mixed reactions; some people criticize the poor technical quality of the graphics, while others are amazed at the amount of detail put into making Liberty City a lifelike environment. My view is that this game doesn't need high polygon count graphics, because the cartoonish style of graphics fits perfectly with the game's nature. Every area of Liberty City and the several actions that you perform through the game are packed with tiny details that make everything feel more lifelike. A few examples would be the way your character has to stop and takes his breath after sprinting, the damage modeling of the cars, the various signs on buildings and scattered all over the city, etc. The visuals in this game are truly top notch.
Another area that potentially steals the show is the game's audio, or more specifically it's soundtrack. As you cruise around Liberty City, you have a wide variety of different radio stations to choose from, each packed with some great music and hilarious commercials. The best radio station however would be Chatterbox, a laugh out loud talk radio station, that not only serves for laughs, but for understanding some of the game's characters even better.
There's no doubt about it, GTA III is a legendary game. Even years after it's original release the game is still just as fun to play and can hold it's own against any of today's games (including GTA Vice City and GTA San Andreas). If you're new to this series, this is the one you want to start with. Everyone else consider this review a tribute to the masterpiece that is Grand Theft Auto III.