I'm looking for a well-maintained Java bytecode manipulation library with an intuitive API. There seem to be quite a lot of them out there. Any suggestions on which ones to try?
The best answer to your question will be governed by your specific needs and objectives; if you can expand upon what you're trying to accomplish, I can perhaps offer a more tailored response.
In lieu of that, however, in my experience, ASM offers probably the best combination of maturity, flexibility, and ease-of-use:
N.B. The comment @Yuri left below describes a situation I personally haven't yet encountered but that may potentially pose a significant obstacle. Keep his observation/experience in mind while evaluating the various bytecode manipulation libraries.