I have heard of potatoes, celery, scallion. What else do people use for Karpas? Is there one item that is more L'Chatchila than the others?
Another one popularly used is onion. Per this answer to a previous question, there are also people who use bananas.
I can't believe nobody has mentioned the controversy with parsley yet. Many, many people use parsley, but several Aharonim/Poskim have criticized this, because they hold that Karpas should have a Berachah of Borei Peri HaAdamah, whereas parsley may only warrant a SheHaKol as an herb. http://www.raananakollel.org/articles_pesach_halachot_of_pesach.html
Most understand it to mean parsley or celery but others hold it derives from any harvest food of the ground. The truth is that the halachic sources speak about the same issues regarding the alleged Greek or Persian roots of the word or the common root of both of them.