If you would like to watch intire Budapest from above, and especially if you love to shoot panormic photos - You just can't miss this one. Totally free and totally amazing.
Since the Metro 4 is open, it's very easy to get there - Just take it to Moricz Zsigmond Korter station, and than get on bus no. 27 up to the Citadella. Then you have a 3 minutes walking for this amazing view.
I recommend to go there twice - once in day light, and the other after dark, when you get to see the whole city of lights. Just remember that the bus frequency is lower at night, and you just might wait for 0-25 minutes for the next one.