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I don't know if you'd call rescue a specialty course, but for me that was by fun the funnest.
If you have any interest in moving on to Dive Master, look at what specialties are required for that as a guide.
Off the top of my head, PADI requires as a pre-requisite Night and Limited Vis, Deep and Navigation.
If you live in Australia do you need Dry Suit?
I think night diving was fun-it was part of my AOW and I enjoyed that dive the most out of what was taught in that course.
Deep for your insurance company