The highest damage on a rare is probably is combination of two prefices:
1) Cruel: 200% - 300% ED
2) Master's: 100 - 150% ED
That'll give you a maximum of 450% ED on a weapon, that can potentially also be Ethereal. The odd of this happening in the game are astronomical. I hate to say it but if you have a Zod (you'll need one for a 450% ED Ethereal rare anyways), you best bet is to make a BotD and hope for close to 400% ED. The other mods on the BotD will probably edge out any rare weapon.