It's extremely hard to narrow this down to just one, I know. Vince Carter had a ton of ridiculous ones, most notably when he jumped over the 7' French player in the Olympics. But, for whatever reason, my favorite in-game dunk of all time is Tom Chambers on Mark Jackson:
When you watch it in slo-mo, you can see he continues rising even after contact. He took off from the foul line circle and by the time he dunks it, his head looks to be above the rim.
It doesn't have to be the most physically amazing dunk, but what dunk resonates in your mind?