Everyone has a favorite! Meads Bay can be very rough and gets deep quickly but a magnificent beach for walking. Shoal Bay East is also gorgeous with silver-white powdery sand and is reef-protected so it's shallower and calmer. Rendezvous Bay is one of our favorites and no matter where we stay, we always go there 4-5 times a week. Maunday's Bay at Cap Juluca is just a perfect crescent with perfect water all the time. Shoal Bay West is fabulous -- if you eat lunch at Trattoria, you get the use of their lounge chairs for the afternoon -- beautiful views of St. Martin.
Different beaches are calmer and/or rougher at different times of the year. We go in November and the Atlantic beaches (Meads, in particular) can get rough because of hurricanes churning in the Atlantic hundreds of miles away. The Caribbean-side beaches sometimes get windy in January/February. June should be perfect everywhere!