So many good ones...I'd probably say Vince Carter. That one against France is the all-time best dunk, IMO. But, he had lots of ridiculous dunks while playing for UNC and Toronto.
The classic ones are Dr.J, Wilkins, and Jordan.
There's been plenty of good ones in recent memory. LeBron, Giannis, Jason Richardson, Zach LaVine, Blake Griffin, Russell Westbrook, Aaron Gordon, Larry Nance Jr, Gerald Green, James White, Nate Robinson, Zion. This kid on Baylor, Kendall Brown, is consistently getting his head above the rim. He's just not overly creative with his dunks, but he has potential to be an all-time great dunker.
Some of the professional dunkers are incredibly impressive. Many remember Jordan Kilganon from the All-Star game a few years back.
I don't know how this dunk is even possible.
Kadour Ziani, who stands at about 5'10, allegedly had a 56 inch vertical jump.