Slow motion/bullet time is a mechanic used in many games to great effect; what do you think is it's coolest implementation, and why?

There is only one obvious answer Superhot. Especially in VR it's so cool to determine the speed with how slow you move yourself.

In modern games I like the auto slow motion for example when drawing a bow, you get a chance to aim while making sense of the chaos. It's usually under accessibility settings, appreciated and fun to use.

Yeah - by rule, I pretty much never go with statements like "obvious answer" or similar, but Superhot is just that rare exception.

Second place goes to Max Payne, for popularizing bullet time in games.

I supposed Super Hot built an entire game around it.  

Bayonetta again built around it.

It looks dated now but at the time Shenmue's use of it in a cutscene was impressive in 1999. Iwao fighting Lan Di in the opening of the game.

Viewtiful Joe is the best at slow mo and super speed.