Diana will win. Thanos had trouble keeping up with Spiderman. He also almost got killed by his daughter who fights with knives. Diana has the feats to cut or stab thanos. Thanos will be unable to one-shot or even kill her unless he constantly pounds her with punches.
First of all Thanos only struggled against Spider-Man due to the portals created by Doctor Strange which allows nigh instantaneous travel. Second, Diana has impressive reactions eped butlacks the combat speed to contend with Thanos, who speed blitzed Hulk, who in turn blitzed someone who can casually react to an RPG.