Basically what the title says.
My favorite is the Conformer. Reasons Being
1) It does high damage to any ememy decently lvled.
2) It does it's regular damage when you use Morph.(not sure why I like this so much but I do)
3) It doesn't lose damage as you lose Hp/Mp (ruby and emerald can both sap those stats)
4)My fav character equips it :P
Oh, and if you feel like you can post which is the worst ultimate weapon. Personally I think the higher hp based ones are the worst... now if they powered up as you lost hp I would like them more... but as it is I find them quite useless in the battles that I would actually want them the most.