I haven't been playing too much of the single player, but I have played lots of online. I absolutely love Cops 'n Crooks and Bomb the Base II. I've already been to the 10th rank twice, but it reset both times.
When I am in single player, one thing I love to do is watch the TV. Don't ask me why, but those shows are very entertaining to me. The one that's like the History Channel that gives a parodic history of Liberty City that's almost like the real history of New York is awesome.
This has to be the deepest game I've played ever besides maybe San Andreas, but that was just because of sheer size, this one feels deeper because the level of detail like you said Des.