I love GTA3, I played it through about 3 times after-each-other, just to try out as many tactics and ways of completing missions as I could. I did 100% completion two times, that's a great way of testing your skills and getting the very most out of the game...
I'm currently playing Vice City through for the third time, going for 100% completion for the second time. I can't wait for GTA4 (or San Andreas or whatever it ends up being called)