We stayed at the Marriott Grand Flora in March, and had no complaints (other than the price ). It is well located at the top of the Via Veneto overlooking at park (? Borghesse ?). I remember the Hilton being described as somewhat isolated from the sightseeing part of town (which was our main interest), and I no information about the Intercontinental.
P. S.: The only creepy thing about the Marriot Grand Flora is that the building was once an SS headquarters during the war.