It really depends on what your expectations and standards are. There's a wide range in the resort, so please just take this as my opinion and no more or less valid than anyone else's.
I have to agree, El Faro is a great restaurant and the food and service is out of this world, but isn't what I would call a 'special' place as it lacks a little atmosphere. Also, I am not a fan of the Piramide d'Arona.... we've stayed in the Mare Nostrum once or twice a year for the past seven years and usually get a freebie there. Again, it's OK, but it's not special, and sometimes it feels a little kitsch. Maybe we've just overdone it. Although El Faro and the Piramide would be a good night, for me it would not be a great one, and 25 years deserves a great one!
IMHO, if you are after a special evening with excellent food and wine, lots of romantic atmosphere and music whilst you eat (usually a violinist playing the classics, and a tenor singer that could come straight out of Il Divo), dancing (the romantic sort) and very special service, then I would check out El Molino Blanco (the white windmill) in San Eugenio Alto. It is just fabulous for this sort of occasion. Meal for two with Champagne should be around £150-ish, but worth every penny.
El Patio on the seafront in the Jardin Tropical was also fabulous in its day, but again not quite so romantic as the above. I've also heard that it was closed down, so maybe that's not an option.
Alternatively, for great food but a little less atmosphere, there is an excellent restaurant in the Bahia Del Duque. Nice place too.
But if you really want to push the boat out I would do the following....
Go into the Cleopatra in the Mare Nostrum and ask to speak to the restaurant manager in the club house. Tell them that it is a very special occasion, and ask them if they could organise a table on the beach for you and your other half, complete with torches and waiter service from their restaurant. And then push the boat out with the food and wine. I've seen them do this before, although for people staying in the hotel. It looked the real deal!
Good luck!