The answer is an emphatic Grand Theft Auto 4. The game does not merely excel in one area it excels in all the relevant areas. The single player mode has depth in spades. A good storyline, decent dialogue, humor in spades, and has more then enough game play mechanics for any three games on the market. All of which is pulled off flawlessly.
That would be enough to warrant such acclaim. However Grand Theft Auto has a massive world to explore, has an almost infinite variety of side quests. Which based on the games design can actually be played in various fashions. With hundreds of secrets to find. The game can easily mount upwards of over a hundred hours. This all before you get into the online modes.
The game positively shines online how many games have online modes where you may spend days just tooling around doing your own thing. The game allows for player control of all features, and lets be honest how many games let you have a sixteen man bull rush on NPC bots. Get sixteen guys together keep the cops on, and see how long you can hold out. Remember everyone pick up a rocket launcher those copters are real killers, and remember last man standing has to try to escape.