Brown Derby, hands down! With the exception of the Brown Derby, the restaurants at MGM do not rise above the level of "Ordinary". The Brown Derby on the other hand has excellent food and service, and is actually worth eating at, IMO.
This does not appear to be an option on this poll however, so I did not vote. Frankly, I'd just as soon eat at Spoodles on the Boardwalk or one of the Swan/Dolphin restaurants rather than the one listed above, but that's just me. Lot's of folks like the Sci Fi, but not for us. H&V is a cafeteria style place. 50s prime time seems fun, but go for ther enterntainment, as the food is pretty ordinary. Mama Melros is just OK, IMO.
of course, YMMV.