My absolute favorite for dandelions is to pick them young (slightly less bitter) and then use them as a salad, with simply oil and vinegar, and lots of bread. It is one of the most delicious salads, and very good after a heavy meal. I remember eating it AS a meal, with plenty of fresh italian bread to dunk in the oil and vinegar that remains at the bottom of the bowl.
They are actually far less bitter than chicory, which is very popular in Italy - either boiled and dressed with oil and lemon as Koukouvagia says, or passed in the frying pan with sauteed garlic - if you get it here you get cicoria all'agro or cicoria ripassata (chee-KO-ree-ah ahl AH-groh ---ree-pahs-SAH-tah)
I find chicory a little too bitter for me, but i love the slightly bitterness of dandelion. Never had dandelion cooked, though.
The older and larger the leaves, the more bitter it is.
A strong memory of my childhood is older italian women with a small knife picking the dandelions from the lawns, putting them in their apron that they hitched up to make a sort of sack. This was in the states. You might still see this here in italy in the country.
Unfortunately with the level of pollution in Rome i don;t really trust the dandelions i could find here in parks and along the roads. Only recently was unleaded gasoline introduced.