This has become one of the top trading platforms for digital coins in existence, and Bittrex supports a pretty large pool of different alt coins. The user interface is easy to get a hang of, and also boasts highly-regarded safety features. A lot of cryptocurrency traders go to Bittrex simply because of the name recognition.
Image source: bittrex
Pros: Hundreds of altcoins, top-level security, easy to use, reputation
Cons: Slightly elevated fees, not the best customer service