Just about everone who boats has a favorite island or two. How about
everone listing some of theirs, along with some high points?

I'll start:

Northeast: Block Island and Martha's Vineyard. Both are convenient
to Long Island Sound and the NY/CT metro areas. Both have great
scenery and well protected harbors. Both have good fishing and
supplies.

Caribbean: St Martin's, Antigua and Martinique - all for slightly
different reasons but all have good harbors, scenery, beaches and
ambience.