Thanos could hold them off by himself and might eventually win simply because he can hurt any of them and none of them have the raw power needed to hurt him. Has anyone seen what happened to Thanos when the Silver Surfer decided to go all out and blast him with all the power cosmic he could muster?
Warrior madness Thor with power gem was stalemated by Thanos before he properly took advantage of the power gem. We also do not know if Thanos was getting boosted using outside means although I would like to think he wasn't. A correctly written Thanos should have no problem keeping up with Thor at the stage he was in. However, Thor was continuing to grow in power and had he got loose he could well have killed them all. So we really do not know how powerful Thor would have become once he fully tapped into the power gem.
We only know that every time he realized he needed more power (such as after Beta Ray Bill beat him down) he was able to get it from his own mind even without the power gem which is what enabled him to almost kill everyone else in short time and handle both Power Gem Drax (high class 100) and Maxam (high class 100) simultaneously and take the power gem from drax by force.
Given that a perfectly sane Thor does have the raw power to beat down Thanos (only if Thanos is caught unawares) a truely insane Thor even half tapped into the power gem would probably kill Thanos and the JLA.