Emerald is the easiest of those listed...
7x counterattacks and a +HP on everyone with nothing else so his stomp (its been a while, no idea what its actually called) only hits for 8888 and you can just attack out. Theres also a 20 something second method with barret's ungarmax + a hero drink.
IMO Ruby/Omega 8/Omega 12 were all equally hard with no preparation, but if you took some time to set things up they're all pretty easy. Going to give it to Omega 12 just because of the time it takes to get to him