Considering that they start 4 kilometers away from each other, nothing prevents Shigaraki from using Omni-Factor Unleash + a Decay wave that wipes out entire cities. Even the strongest Avengers would have a hard time getting close.
I can only see Iron Man, Thor, War Machine, and Captain Marvel surviving that first Decay wave, and they still would have a extremely hard time getting through
Shigaraki's myriad Quirks.
Just looking at the Avengers' profiles, they don't really have anything to contain Shigaraki, whose lifting strength is far above that of Thanos.
The only thing that could pose a mortal threat to Shigaraki are Thor's 5-C attacks with Stormbreaker, though that doesn't mean that his attacks can one-shot Shigaraki, no. Even if Thor can somehow get close to Shiggy and Shiggy can't dodge his attack, cutting off Shigaraki's head like he did to Thanos wouldn't be enough to kill him, Shigaraki would simply regenerate his entire body quickly.
Thor would need an attack that would destroy a large portion of Shigaraki's brain, which would be quite difficult with his weapon.
So I'm voting for Shigaraki, most of the Avengers would die at the first wave of Decay and the surviving ones would have an incredible hard time even getting close to Shigaraki, and those Avengers with ranged attacks don't have anything that could one-shot Shigaraki and bypass his regeneration, while Shigaraki only needs to keep spamming his attacks until they all die, the Avengers cannot outlast Shigaraki in a battle of attrition.
