My favorite SHA256 altcoin is Unobtanium (UNO) because of the potential value over time due to limited supply.
My favorite POST altcoin is VeriCoin (VRC) because of the implementation of Proof of Stake over Time, allowing anyone to profitably stake (mint) coins regardless of wallet size.
My favorite POS altcoin is Exclusive Coin (EXCL) because of the potential value over time due to limited supply.
My favorite Platform-based altcoin is STEEM (STEEM) because of the integration of rewarding social media. Kinda like a blockchain version of WordPress.