Coinmarketcap is the best and most famous, but I would like to add one alternative to Coinmarketcap:
LiveCoinWatch It tracks in real time (with historical info) more than 1,000 altcoins as well as their volume on more than 100 exchanges. I like the design and especially the ease of use. If you hold a lot of coins, you can have an overview of your entire portfolio on their site.