Oh ceramic knives are the best. They're so easy to clean, and they stay sharp for so long. They're a bit more brittle than their steel counter parts, but they cut so well, as long as you treat them right, they'll treat you right.
These models come in a variety of lengths and sizes. Personally, I like a 4" blade if I only have to choose one. It's compact, but still cuts through most citrus just fine. But then a 6" blade is better if you like to use a lot of oranges. I could find a use for all of them, I have no doubt.
Please note that all products on Obarlo.com ship individually, and can have shipping times of up to 6 weeks