The most exact answer to your question would probably be the triangle, but I doubt you're looking to improve your triangle skills
If you want to understand them all to a degree, I would recommend the piano - it does require patience, but it's a great teacher to learn other instruments as well.
Rhythm guitar would be the most beginner friendly, I guess - but you're not going to learn much theory this way, and guitar skills are harder to translate to another instrument.
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