If you must go to Maremagnum (handy for clothes shopping on a Sunday), then treat yourself to lunch at Elx, it's the only establishment worth going for.
Elx delivers traditional Valencian cuisine to a very high standard (I have family from Valencia and even they approve) in a modern but classy atmosphere. It is all about fish, paellas and white wine. Everything worth trying.
The best:
The quality of the fish and the care and attention given in its preparation.
The worst:
It's in Maremagnum!