Probably the best of these is Machroes on the Llyn Peninsula, where you'll find millions of shells with a wide selection of species. Tower shells are most abundant.
The shoreline at Ffynnongroew has millions of small shells - mostly cockles, but worth a visit if in the area. The south-western coast of Anglesey is good for variety of species, with Malltraeth Bay (the western end) and Porth Lleidiog being good choices.
Shell Island in Gwynedd seems very over-rated, although it's claimed that over 200 species have been found there.
In Ceredigion, the North Point at Ynyslas is good at low tide where you'll find smooth clams and Iceland Cyprina. Note that shells of the unexploded variety are also found here from time to time,
so be careful what you dig up. At Bullwell Bay in Pembrokeshire and Neyland on the opposite side of the haven there is good variety and quantity,
but slipper limpets are the most numerous. At Ginst Point and Tywyn Point in Carmarthenshire you'll find millions of shells (mostly pod razors, otter shells and cockles) but unlikely to find anything rare.