Depends.. with a good strategy, they are all easy (Emerald and Ruby that is, Ultima is always a joke
). I mean, with KOTR, W-Summon and Mime they both go down rather easily (add in Hades for Ruby to stop him from countering KOTR).. so I thought of what they would be like to beat without KOTR. I've even beaten Emerald without using KOTR.. you can do pretty nice damage to him with Demi and limit breaks, but Ruby is an entirely different story. There you have to go with Bahamut ZERO I guess as the only option.. I've never tried to beat Ruby without KOTR, but now that I think about it, I'm definitely going to try next time I play FF7
Anyway.. I voted for Ruby.