The award for best altcoin exchange as of this writing must go to US-based exchange Bittrex.
Built and operated in the United States, Bittrex is the next generation crypto trading platform created by security professionals. The founders are top-level security specialists with backgrounds at companies that include Blackberry, Microsoft, and Amazon. Wow!
When virtual currencies take off (and they will), solid trustworthy infrastructure is a must. You would be hard-pressed to find a more qualified team than the experienced crew guiding Bittrex