If voting is still open, I'm gonna vote Darkrai. Wasn't it detailed above it's naturally defensive & wary earlier? Also, wasn't it mentioned it was skilled enough to fight Dialga & Palkia, or at least keep up with them?
Add to this Danmaku, Non-Physical Interaction for any Pokemon being able to hit ghosts meaning phasing might not help Shadow, & Darkrai can probably hit & avoid him.
Also, as Cal requested above, proof that Shadow goes full fights with opponents on his level of speed without getting hit? I'd assume Gun Soldiers aren't really Shadow's Level, & he's been beaten by at least Sonic & Infinite, getting hit in their fights, no?
AFAIK, he also doesn't have reason to know Dark Void will put him to sleep if he touches it; If Shadow tries to tank it or go rush through it, he might get affected anyway.
Also, Darkrai has:
Resistance to
The Creation Trio's abilities (Can take some minor hits from, and interact with Palkia and Dialga, and survived Palkia's pissed off attack)
This would include Time Manipulation (Dialga) & Spatial Manipulation (Palkia), so how sure are we that Chaos Control would work on Darkrai?
Heck, how does Darkrai's Power Nullification work? Also, its BFR? (Although, Shadow has Dimensional Travel, so how does that work?)
Also, I'm surprised nobody mentioned this, but Darkrai has Telepathy & Precognition. How do those work? Could it be reading Shadow's thoughts mid-combat as he strategizes & realize he's going to stop time/teleport/use chaos control, etc.?
Could its Precognition foresee Shadow one-shotting it if he hit Darkrai? That'd be a good reason for it to get very avoidant. Afterimages & potentially Spatial Manipulation (Assuming it's not just Spatial Rend.) as well as Portal Creation could help.
There's also the matter of Time Travel. Both can do it, but how does it work? Can one follow the other? Are they likely to know where the other's going?
Come to think of it, is Time Travel even useful? SBA says this is in Central Park, New York, so, AFAIK, their younger selves aren't gonna be in this timeline to time travel to & kill.