Grand Theft Auto III
Though it is shorter than its followers, GTA3 has its fair share of extra stuff too: rampages, vigilante/rescue/taxi missions, optional mission chains, stunt jumps, et cetera; not that I bothered with almost all of them, but they are there for your inner completionist if you have one. And, in the end, I was pleasantly surprised by the story cohesion - it was still generally weak, but more than I was expecting.
All in all, I'm not sure what more I can say except, it's a fun game that has its problems, much like the sequels.
Progress: Cleaner (1616), finished main missions