The illetes beach is not nicer then any other beach on the island. it is more just another beach on the island with lots of tourists.
Like Poster HARTER says, you can find more empty long streched beaches in the east.
There are some bays worth visiting in the West around Sant Elm, Cala d'en Monjo, Cala Fornells, Cala Marmassen....etcetera but to get there is more complicated.
In general July and for sure august the island is packed on the beaches as it is the only natural way to cool down from the blistering heat.