My fav Dressphere is definately Yuna's special dressphere, it is so cool, and it's attacks all look cool, and it has the Floral fallal ability which is just bitching and king of all pwnage
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Also me likes florial fattal (cant spell it) XD
Both are very well designed and the colours in each outfit are well balanced against her hair, earring and braid.
The Gun Mage is a rather elegant outfit too!
Practically, the Songstress isn't that useful, so I'd have to pick the Gunner for Yuna - her attacks are reasonably powerful. 
I absolutely hate her Songstress uniform.
Practically speaking, the Thief isn't all that powerful, but she's swift. Consequently, Rikku fares better as a Warrior when she's the last standing, but combined with Yuna (Gunner) and Paine (Warrior), her speed is very useful. It certainly helps one build up combo attacks! 
The softer more elegant outfits look incredibly clumsy on her.
The Warrior is also one of the more powerful dress-spheres, so I've never had any qualms with keeping her as a Warrior unless absolutely necessary. 
