My sentimental pick is in III,when Michael flashes back to the dance scene with Appolonia,just before his death.
Although the scene lasts only a few seconds,the expression on his face speaks volumes. We see a happy Mike,a man who still has a chance to take a better path than the one he ultimately chose.
We see the sparkle,the impish grin that must have swept Kay off her feet the first time she met Michael.
The phrase that comes to my mind every time I watch that scene is "What might have been?"