Psychotic - you're very wrong.

(By the way - I would never count Zanmato, The End or Cait Sith's limit break a proper way to defeat an enemy as they are all instant death, even to bosses. Pointless, might as well use a gameshark).

Ruby is a bit tougher than Emerald simply because you have to set the fight up at the beginning cos he removes 2 characters. Still can do it with 1 (as I did) or you can fanny around and save the other 2.

For emerald though here's how to kill him in 1 move:

It takes a lot of levelling up and mastering materia (many hours) but it's very doable.

You don't even need a mastered KOTR.

You need 6 (yes SIX, count em) mimes. 3 final attacks and lots of mastered magic plus materia. Also boosting your magic stat is useful. You'll need w-summon and hp absorb.

Get your magic as high as you can so that KOTR does 13 lots of 9999 damage.

Then simply pair hp absorb with w-summon. Final attack with mime and counter with mime.

Then you've beaten emerald. You initially w-summon KOTR which does 13 x 9999 x 2 damage = 259974 - over a quarter of his total HP. You also gain all your HP back.
If he hits you then you'll counter attack with another 259974 damage. If he kills a character (which he shouldn't if you've got 9999 health) then you'll do another 259974. You only have to do 4 of these to kill him. The only way you can die is if he does Aire Tam Storm which means 9999 to all (materia based) and you're fuggered cos you don't have phoenix on your final attack.

Still - you can kill Emerald by just attacking once. You should get the rest of the hits in from either dying or retaliating.

Easy.

Omega weapon in 8 is also easy, even without heroes. All you need to do is make sure you've junctioned correctly and that squall and zell have auto protect, shell and hast. Have Quistis do mighty guard and cast aura (if you don't get it from mighty guard) on both zell and squall. Zell can do huge amounts of damage each limit break, as can squall. You'll defeat Omega in a couple of hits, before he gets chance to do Terra Break.

The reason I'd say Penance is definately the hardest is that there's NO WAY to kill him easily. You've got to stick at him for AGES before you can kill him. He just takes so long it's not funny. Even if you're as maxxed out as it's possible to be (I nearly was).