Thanos wouldn't be able to defeat them all. They're all on his level. Thanos as well as the others are capable of amplifying their powers to crazy levels.
. I'm not certain that Thanos would survive a blast from Monarch, but Thanos ultimately destroys this guy in every way. Thanos always comes back, which was what we saw when he was transported into null space and forced to commit suicide. Only to come back.
. If the Green Scar is holding back, he doesn't deserve being mentioned. Ultimately, Thanos wins this. If this battle had no time limit, Thanos would be able to come back in the future, and continue his previous work, until he ultimately won. First battle goes to the Green Scar unleashed though, there would be a beating like no one has ever seen.
. Doomsday would have to be dumped into null space for Thanos to ever win. I actually see them stalemating.
Win by KO? Team wins. Alone? Well..