My choice here is XMR (Monero),
http://monero.cc/,
https://bitcointalk.org/index.php?topic=583449.0. This coin has been trading on Poloniex since May 19, 2014 so it meets the three month requirement.
https://poloniex.com/exchange/btc_xmr. It is the leading Cryptonote
https://cryptonote.org/ coin in both market capitalization and volume.
http://coinmarketcap.com/.
It is proof of work coin that offers some very attractive features including robust anonymity using ring signatures, and adaptive limits. The latter means it is not constrained by the fixed limits such as the 1 MB blocksize limit in Bitcoin. It also has a very active development team and is generally considered to have a fair launch. Because of its youth it is also very much a "diamond in the rough' in that it requires a fair amount of development work including the back end database and GUI. This can provide an attractive opportunity for an
very risk tolerant long term investor that is experienced in the crypto currency markets.