We've been on 16 Carnival cruises and 16 different ships. We've had one cruise we weren't really happy with, but that was due to staff problems and they were probably corrected when the ship was repositioned.
My preference is the Spirit Class ships: Spirit, Pride, Legend, Miracle. They seem to be easier to navigate and although they are big they don't seem quite as big somehow. The layout of the Lido deck is also better on these ship.
On the other hand, the Conquest Class ships (Conquest, Glory, Valor, Liberty, Freedom) have a few extras that the others don't, like a special place for Fish & Chips (which also includes other fried seafood). My husband plans on eating there often, we may have to split up since I like to eat lunch in the dining room! 
I have no problem with the Fantasy Class ships (Fantasy, Ecstasy, Fascination, Imagination, Inspiration, Elation, and Paradise) either. (Or any of the other classes I haven't mentioned.)
The decor is more to my taste on some ships than on others. We've found the decor to be one of the "fun" things to see once we board. Carnival, to this point, even includes floors and ceiling when they decorate. There was one ship where I remember sitting and waiting for our number to be called for debarkation, looking up I saw something in the decor I hadn't noticed during the week we were onboard. It was in a place we had gone by or been near often, too.
On some ships you can't go from one end of the ship to the other on the same deck, but they hand out - or have setting around - maps that help you navigate the ship and before you know it you don't need the map anymore. (There are also maps on the wall at the elevators.)
So far we've chosen our cruises based on the ports we want to go to and then look to see what ships we haven't been on yet. We are really excited this year as we have a new port or two AND a new ship!
