Which Legendary Robin is the Best?

+atk, without a color advantage 48 attack neutral is rather low so an attack boon is her best. Avoid lowering her speed as it means she's taking more damage unnecessarily by being doubled.

Without a doubt +attack -res is better. One of Robin's best qualities is her speed, so dropping it is detrimental to her usefulness. Meanwhile, her attack is rather low, so an attack boon is very nice for her. Res isn't a great bane, but you can live with anything that isn't -speed in my opinion.

+ATK/-RES definitely. If you want her to tank mages you'll need the speed anyways.