Loads of votes for celery here. Poor old celery, it's very good for you and has been used in cooking for thousands of years.
I don't eat much of it on it's own but if you destring it and use it as an ingredient it adds huge flavour and depth to all sorts of dishes. Stocks, soups, chilli, pies - they all benefit from celery without adding a distinctive celery taste to the meal. It's one of the main ingredients of cajun and creole cooking. You probably eat loads of it in small doses without realising it's true worth.