For the Federation, I think my favourite would have to the the Luna (Reconnaissance/Mirror Deep Space Science Vessel). With the right layout, she dish out damage better than the Defiants, last longer, and (IMO) look sexier doing it.
For the Klinks, I'd have to go with the Hegh'ta. She looks nasty, and she's got the teeth to back that look up. The universal BO layout is also a plus for me.
That being said, I so want the B'rel Retrofit. Too bad I can't afford it atm. Damn you economy!
Also, was anyone else disappointed with the Odyssey?