I would actually give the JLA the win as well.
Flash is always a problem, since you either lose to that guy or stalemate him by being so powerful that he can't do anything to you.
But besides the ridiculousness that is the Flash, I see Thanos struggeling with the speed advantage of his opponents big time. Like really, really big time.
Thanos does not have a good track record, when it comes to fast characters:
1. Captain Marvel was tagging him at will, while Thanos failed to grab him. Then Captain Marvel made a mistake and flew too slow and too low, which allowed Thanos to grab him and finish the fight.
2. Thanos was unable to catch the Runner, who was running circles around him and then ran away, while Thanos praised his speed.
3. Eros was dramatically outspeeding Thanos, saying that he has a speed advantage over Thanos and Thanos confirmed it.
4. People often try to make an argument for Thanos by providing the scan with the IG and the Silver Surfer, but the panels also state that Silver Surfer simply missed his target and then we see Thanos realizing that he is losing his cool and acknowledging that he nearly lost the gauntlet.
Thanos is a slow brawling brick, which is perfectly fine. If he tags you, you're in a world of trouble, but in theory he should not be able to tag half of the JLA, due to their natural speed advantage, if fighting at their best.
The only way Thanos can win it, is if he sticks behind his force fields (the ones, which protected him from Omega) and plays it save with energy beams and matter manipulation, but the Flash might be able to come up with PIS powers to ... I dunno, phase through the force fields and speed-steal him? -____-"
Thanos uses his shields like he did against the Fallen One. He has also grown in power since then and has other abilities such as mind control or force block.
Thanos will fight effectively and quickly adapt. It's who he is. Always thinking quickly on his feet and always ahead of his foes. The Jla is in trouble as they normally are against weaker, dumber bricks.
__________________ QUANCHI112:In between the passes Khan will tear out the orca teeth and use them as an offensive weapon. Khan has crushed a skull before so tearing a tooth off a whale should be no issue.
that was Thanos before upgraded his powers vs captain Marvell
Runner was using the Space gem to alter Space as well as using his speed
Eros fought a weaker clone t
Not the real Thanos.
Thanos has tagged the Fallen One and Genis Vell at flight speed and also Ganymede who speed blitzed Surfer and a Tyrant robot, to name but a few reaction feats
While the no prep stipulation for Thanos favors the JLA, the basic knowledge stip heavily favors the Titan. The abilities and weaknesses of many of the JLA members i.e Superman's weakness to kryptonite and red solar radiation, Flash's use of the Speed Force etc are widely known among the general population of Earth. Thanos is on the other hand is unknown to the general population. The JLA would know nothing of Thanos' would know nothing of his shields or energy manipulation abilities. The League's best chance for victory would be for Superman to go all out, using maximum speed and strength, while Flash hits him with a barrage of IMP's to overwhelm and quickly subdue him. Neither of those tactics would be in character however. Superman for example doesn't go all out against an opponent unless he is absolutely sure that doing so won't kill them. Once the fight starts most of the League's weaker members can be taken out with one omni-blast, leaving only the most powerful members to face Thanos. That slight delay I mentioned earlier is all Thanos needs to seal the deal.