I have stayed at Occidental Grand, Occidental Allegro and Fiesta Americana (pre-Wilma).
Pure beach/pool time, I would choose Grand, Allegro, then Fiesta Americana (in that order).
For diving, each has their advantages: Grand and Allegro are closest to the southern reefs and dive ops pick you up at hotel pier, so short ride to the dive sites. FA: also pick-up at the pier but a little longer ride to the southern reefs (where most of the "first" dives take place).
Price: Grand, Allegro, FA (highest to lowest).
Snorkeling off the beach: FA, Allegro, Grand (best to worst).
Rich