I was thinking of finding a new hobby, these all sound interesting.
Which is best for a beginner?
Depends what you want to learn.
If you want to learn how to dodge haymakers and beat up the average joe with your bare hands, boxing. The cons are eventually, after you've got in boxing condition, you will get punched in the face quite a bit which may discourage you, especially if you get hit, very hard.
With muay thai you are conditioned to learn how to take a lot of pain and give a lot of pain. The cons are similiar to boxing. In this art you will be kicked and also punched. You probably wont take as many head shots but being punched in the face and kicked everywhere is not fun for a lot of people. You will primarily be throwing punches, elbows, knees and kicks about the body, leg and face region. The physical conditioning and ability to give out a lot of damage is a plus.
With brazillian jiujitsu you will be strictly grappling. You will learn how to take someone to the ground where you will then learn how to put them in a position where you can choke them with your arms/legs or break any bone on their body with your hands/arms/legs. You will also learn how to escape any any every lock, hold or grasp known to man and will have answers for any situtaion where someone attempts to grapple, choke or break bones on your body with their arms and legs. You will not be punched or kicked although you will experience a lot of pressure under the weight of someone else. The cons are, if you do not tap, someone could break your arm/leg/shoulder/ankle/wrist and you also run the risk of injuring yourself in some kind of freak transitional accident. Injuries to the fingers, toes, knees, acl, and elbow are common (depending on how competitive you become) If you're just there to have fun in the academy and be a dojo warrrior this risk is significantly lower.
Either way all 3 arts put you above the physical capabilities of the average man and all 3 practioners who are in top shape bag dimes so not only are you in shape and a physical badass, you will also command the respect of women who will obey you naturally, because of your genetic superiority over the weak.