I don't find anything in GTA as fun as JC2. There's nothing you can do in GTA4 that you can't do in JC2.
And JC2 has better mechanics hands down dude, what the fuck. GTA4 is very clunky when it comes to moving around, that's always been a criticism of the series. The driving in GTA4 is far too difficult as well, whereas JC2's is somewhere between Saints Row and GTA.