Do not look down on the awesomeness that is Ultima Weapon! Omega might have around 9 times as much HP, but Ultima Weapon's look, atmosphere and sheer badassness far surpasses that of Omega! Plus Ultima got a huge freaking sword. Omega does not.

...besides, Omega is a pushover as well. Every single enemy in FFVIII is thanks to the broken junction and limit break systems.