A bit late for advice, but my personal favorite is Fatal, in an alley off Vaci Ucta. It is definatly the only place on Vaci Utca worth spending your forints at (the rest are hopeless rip-offs). The food is good and the portions make it worth it.

I don't know if I could recommend Langos to everyone, it is basically fried bread topped with sour cream and cheese, even too rich and filling for my own grease-and-fat loving self. It is very Hungarian though, but I would say there are more 'Gyros' (Doner Kebab) places in Budapest than Langos places now.