I'm often asked by developers who are new to the platform what kinds of thing they should write unit tests for. In general, I find that question rather frustrating. Many factors go into answering that question, and without knowing the specifics of your org, it's impossible to answer comprehensively. On the other hand, it's much easier to answer the question what not to test? While the answer to that question isn't cut and dry either, it's at least simpler to provide guidelines for. With that in mind, here are some things you should most likely not test.
Managed packages have to provide their own unit tests. Additionally, you don't get to see the code from managed packages, so writing unit tests for managed package code is not only difficult, it's fruitlessly redundant as well. However, this isn't carte blanche for not testing your code that interacts with the managed package code. The only time I write unit tests for managed package code, is when I'm...