Go to the Buda side of the river and position yourself across from the Parliament. Try golden hour, blue hour and night. Better yet, get in a boat and try a shot from the Danube.
You can make a photo like this from across the river but the lower angle from being on the water is a little more unique (this building gets photographed a lot).
The Parliament building is one of several iconic sites from the Danube River in Budapest. Many photos of this building have a smooth river / reflection either on a calm day or from motion blur. I wanted the water to be more alive and show the abstract light reflection instead of a more accurate building reflection.