I've stayed at the Full Service, Courtyard, and Boscolo properties in Budapest. They are all great and you can't go wrong.

FS Marriott - Probably best location. Nice lounge with amazing views.
Courtyard - I remember it seeming very new and had a great breakfast. It is probably the least expensive of the properties as well.
Boscolo - By far the nicest. As a Platinum I was given an amazing room probably one of the nicest upgrades I have had with Marriott.