Been to all three... I'd pick Brown Derby for the fantasmic package - dined there twice in Dec. 2010 and the food was great. BUT, it is more expensive or 2 dining plan credits. If I wanted a less expensive dinner or to only use 1 credit, Mama Melrose is good... not great.. but good enough, went there Oct. 2009.
Hollywood and Vine for dinner --- a big NO, the food in 2008 was the quality of a very cheap budget buffet, lunch in 2009 was the same, and the current reviews sound like it didn't get any better. BUT, we did enjoy the character breakfast there in Dec. 2010.
On our Dec. 2010 trip, we really liked 50s Prime Time Cafe..... so we will have to dine there on our next trip. 50s PT is not a fantasmic package offering.... BUT, if I was only going to Hollywood Studios for 1 day, I'd probably go to 50s PT for lunch and just wait it in line for Fantasmic....
BUT, if I was going to be there during a very crowded time of year (which I won't), and only going to HS for 1 day.... I'd probably just do Brown Derby fantasmic package since the line for fantasmic could be long without the package... so I hear....