The code that you use to test should test the features available the in JDK used to compile the target class. For example, if you know that your target is written in Java 1.5, it is reasonable to assume that the code might include generics, so you will want to make sure that your chosen decompiler handles them properly. In my experience, the freely available decompilers tend to lag behind the JDK releases in terms of the features they support by 1-2 releases.
Based on personal trial and error, JD tends to do the best job in general. However, if you're decompiling code that was written in 1.3 or lower, I'd also suggest you give JODE a try.
EDIT, 5 years later:
CFR, Procyon, and Fernflower lead the way in this space.
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