I'll toss in my 2 psi ...
Rescue ... to my concern, this is the most important continuing education class you can take. Yes, the focus of the class is generally on assisting others with problems, but the class is also about helping you understand and learn how to keep yourself out of being that person who needs assistance. For that reason alone I think it should be taken as soon after OW class as possible.
Peak Performance Buoyancy ... the value of this class depends entirely on the quality of the instructor teaching it. It can be a terrific class that focuses on helping you get your bouyancy, trim and weighting adjusted properly and helps you better understand the concepts of good buoyancy control ... or it can be just another "checklist" class where you go through the motions of the exercises without really understanding what you're doing or why. It all depends on how well the class is presented to you by the instructor. My suggestion would be to seek out an instructor who has sufficient control and experience to teach the class properly. One clue ... watch the instructor dive. If the instructor is swimming in a feet-down position, or if they either go to their knees or allow you to for any reason, they will not give you sufficient value in this class for the effort and money you'll be putting into it.
Navigation ... once again, it depends on how it's taught. If the class only trains you how to read a compass and count fin kicks, it's useless. If it focuses on the mental aspects of monitoring your time, depth, direction, and awareness of your surroundings as you dive, then you're looking at a class that's worth taking. As someone mentioned, where you dive people don't follow dive guides ... you'll be expected to find your own way back to boat or shore. On some dives, current can radically affect your navigation ... and that needs to be factored into the class. Once again, the value you get from this class will be entirely dependent on whether you get an instructor who just "follows the book" or one who will put real effort into teaching you the little things that will help you put what the book teaches into real-world practice.
Nitrox ... this is a useful class only if you find yourself pushing the limits of your no-decompression limits on your dives. Most new divers ... particularly those using AL80's (aka "oversized beer cans) ... will be limited by their air supply, not their NDL. In this case, nitrox won't offer you a lot of benefit. On the other hand, if you're using larger tanks and doing dives that force you to end the dive with large reserves, or head to shallower depths before you really want to because you're pushing your no-deco limit, then a nitrox class will have value.
Another suggestion would be to look into one of the entry-level offerings by either GUE or UTD. These are more geared toward people who want to dive a specific way and with specific equipment, but the entry-level classes offer some methods for buoyancy control and air management that can be applied to pretty much any style of diving. They have, for many, proved to be rather eye-opening in terms of what your OW instructor didn't tell you about scuba diving.
... Bob (Grateful Diver)