Hey, this is very personal as some people dislike rocky beaches, but my favorite beach in Croatia is sandy beach on Island Lopud. There arent many places that have sandy beaches but my favorite locations are Korcula, Island Rab, Island Pag, Brac has amazing amount of pebbly beaches great for kids and families. I would strongly suggest checking the articles below to decide what type of beaches would interest you but if you will be staying in Dubrovnik going to Lopud for the sandy beaches is really a must.

Best Beaches to Visit in Croatia (Most popular beaches in Croatia)

Here are articles about beaches for specific locations:

Beaches on Hvar Island: Best Beaches Close to Hvar Town

Best Beaches on Brac Island

Krk Island Best Beaches: Guide to Best Beaches on Krk

Losinj Beaches: Guide to Best Beaches on Island Losinj

Best beaches on Lopud, Croatia (Guide)

10 Best Beaches to visit in Dubrovnik

Top 5 Beaches in Split

The southern part of Croatia has better beaches and wider range, sandy, pebbly while Istria has mostly rocky beaches and some pebbly.