I usually use 'Euagent' who have provided reasonable boats and no problems. It is a big agent, but they appear to have high standards. I have chartered five times with them with no problem.
Agree the comments about Gulf of Kvarner. I had an ugly 7 hours crossing it once.
My favourite place is the island sometimes known as Parmizana or St Klement, the island opposite Hvar town. Park on the south side rather than the marina if you can, tying up to a tree. Find the restaurant of the same name which is nearby, or walk the length of the island with a torch to find an uncharted restaurant that is waiting for me next time that looked promising.