Post Crisis JLA
Scenario 1: The league has had 1 year to adapt to using the gems. Thanos appears in DC universe and goes on Thanks quest to get the gems.
Scenario 2: Thanos VS all of them in a fight. No prep or practice.
Flash with Time gem
Superman with Power gem
MM with Mind Gem
WW with Soul Gem
Aquaman with Reality Gem
Green Lantern with Space Gem