Well first thing to pop into my mind is Ethereum, but i could talk all day about different projects, ICO's, forks etc.
Ethereum has changed so much already that i'am hestitant to call it an altcoin, even though it's not an derogatory term. In my mind there are Bitcoin and Ethereum, the rest is altcoins:D.
But the one i'am most hopefull about is ofcourse Steemit! It has proved to have a winning concept that could alter the way we consume media. In some ways it already has.
Here are a few altcoins iam looking closer into in 2018:
EOS
NEO
Cardano
IOTA