Originally posted on 05/25/2017:
Heh, there's no single answer to that. If you mean what's the 2nd BIGGEST crypto coin, that's Ethereum (ETC), a coin whose favorite buzz words are "smart contract." If you're interested, here's the market caps of all 700+ alt coins: https://coinmarketcap.com/currencies/views/all/
As to what has the largest profit potential, I can actually answer that: it's whichever of the small coins manage to get themselves listed on the alt coin stock exchange Poloniex. When a coin gets confirmed for listing on Polo, its value jumps by an average of 130 times (as in + 13,000%), basically overnight.
But unless you can get your hands on that inside information -- btw, a perfectly legal thing to do in crypto, if you can manage it -- then everyone has their own opinion as to which coin's got the best profit potential.
My own largest position is in the hyper anonymous coin Monero (XMR). I like its small float, perfect anonymity, and large toe hold and increasing usage in the 3rd world. SBR's favorite alt-coin seems to be LiteCoin (LTC).
But honestly, your guess is as good as anyone else's.