Which symmetric cipher is best for studying? I would like to study a modern symmetric cipher (I have never studied a cipher before) and I was wondering which would be the best cipher for studying purposes of the following: AES (Rijndael), Blowfish, Twofish, Threefish or Serpent. Basically I am asking which of those ciphers is the most simple to study.
First, it is important to learn the basics behind all symmetric ciphers. You can get this from Handbook of Applied Cryptography, see Chapter 7, especially 7.1, 7.2, 7.3. If you understand those three sections, you will be off on the right foot.

From there, I would suggest just diving right into AES. It isn't that terribly difficult (yes, there are easier ciphers, but the "market penetration" of AES is so great that it is worth the added effort). There should be some good videos online, plus look at this question, the comments, and answer.

From there, I recommend the book "The Design of Rijndael". It was written by the inventors of Rijndael and has some very useful information on why they designed pieces the way they did.

Finally, I recommend using this site as much as possible to help clarify things. If there is something you don't understand as you are learning all this, search on here to see if it has already been answered. Then, if not, ask a specific question on this site.