Gunvolt has a unique scoring system that doesn't entirely go hand-in-hand with speed, but is ridiculously fun when you get a handle on how it works (mainly "don't get hit and go for as many airborne triple kills as possible" but there's other nuances to it). The kicker to me is the game devs seemed to somewhat realize how substantial scores could get because all the ranks up to S are tiered rather close to each other (say, every 100k) but then S+ can be an entire million points after S, but there aren't any additional ranks past that.
And then there's Stratacombs, where S+ starts at 850k and I have a high score over 3mil on it.
I don't think there's really anything I have that I want to speedrun currently.