When you first put GTA IV in your machine and turn it on, after you digest the massive graphical advance from San Andreas, next youll notice the immensely honed physics. Niko actually feels like hes moving through a world with physical laws of causality; that there are resistances all around him which affect him should you move him right or left. In other words, he doesnt just move right or left, he steps right or left, exactly (or near as damn it) as we would if we chose to do the same. Nikos movements change subtly should he be running up or down stairs; combat is breath-taking for how people fall as they die; getting hit by cars or crashing your motorcycle often looks almost real, if not over-exaggerated (hey, it is a video game after all). Now Im not saying that the other GTA games contained bad physics, not at all, Im just saying that GTA IV boats really, really great physics. Again, not the sole reason I consider it the best GTA game, but definitely one of the deciding factors.