According to Zagat, the most romantic Las Vegas restaurants are:
Alize (Palms), Andre's (Monte Carlo), Charlie Palmer (Four Seasons), Eiffel Tower (Paris), Hugo's Cellar (Four Queens), Le Cirque (Bellagio), Michael's (Barbary Coast), Mistral (Hilton), NOBHILL (MGM Grand), Pearl (MGM Grand), Picasso (Bellagio), and Top of World (Stratosphere).

From this list, Le Cirque, NOBHILL, Michaels, and Picasso have the best food rating, so you might want to go with one of these picks.