There are too many to name, but it's hard to pick one of the "in the moment" runs I have seen, whether it be The Might Bill's SOR4 world records or MitchFlowerPower's SMB3 world records, because each of them can indeed be optomised even further. Whereas something like dam 52 is bordering on, if not outright, perfection.
I could probably name many impressive, high stakes speedruns I love if I really thought about it. But a favorite speedrun that always comes to mind is Puwexil and Essentia's Final Fantasy VI run from SGDQ 2014. I remember watching it live and just feeling so simultaneously relaxed and engaged. It really made me appreciate RPG speedruns and long speedruns in general. I think they can be underrated due to their length, and for not always being constant, heart-pumping action for the audience. But I think they're pretty impressive.
There's a lot of great runs, especially in the GDQ backlog. My favorite record I watched live is probably darbian's 4m 56s 878ms SMB1 run, which ended a month-long campaign wherein he had increased popularity and was facing what at the time seemed an insurmountable barrier. The funniest record I've seen also comes from darbian, although this time a 7m 59s 860ms run of SMB2J. darbian basically manages to accidentally get WR watching an infamous infomercial ("set it and forget it!").
Hard to say, but the first run that came to mind was the first ever 1:09 on Luigi Circuit by Frostbeule/Yazzo