Anywhere between Las Olas and Sunrise if you want be where the main strip area is.
More north you can go to less commercial beaches in Lauderdale By the Sea. That would be Commercial Blvd (ironically) all the way east, and go a little north along the beach.
Just depends where you are staying in Ft. Laud and what you want.
Hollywood Beach isn’t in Ft. Lauderdale, it’s Hollywood, but it’s relatively close.