Bitcoin is my favorite coin although it's not an altcoin.

Out of all the altcoins, I like Nxt because it was written completely from scratch and has quite a lot of interesting and innovative features. I also like Peercoin because it was the first proof-of-stake coin.

Other coins that I like: Dogecoin, Litecoin, Qora, and the three big anon coins (Darkcoin, Bytecoin, and Monero). I might include NEM in the list too once it is fully developed.