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.

Technically not slo mo, but I think it was Braid that also had a level where the enemies move when you move. It made a fun puzzle mechanic.

Burnout 3 Takedown's crash mode had cool slow motion. It was half of the fun to watch the chaos play out in slow motion.

I never played the sequel, but Genji: Dawn of the Samurai was a pretty game on PS2 that was pretty much built around using a slomo mechanic to take down enemies. Even with it the final boss was so fast that you essentially had to parry before he even began his attack.

Not exactly slomo but the tate mechanic in Shinobi (also on PS2) that would freeze enemies in place before stylishly slicing them all open at once was pretty satisfying as well. 

F.E.A.R is probably my pick. Just cool and first time I experienced that mechanic I think.

This is one of my top ones too; the slow motion ballet of bullets and destruction was so epic

Check out Trepang2 as well. Similar to F.E.A.R., but more of a sci-fi setting.