What is in your opinion the best altcoin to invest in for 2015 and why.
Personally I have interest in some older 'proven to be solid-' POS coins with not too high stake, it dillutes the amount of existing coins and it will be harder for the price to rise. So I decided to hold lots of CGB's with only 2% max annual inflation and technically flawless. From the newer coins I got RDD's (5% annual) because it has a great niche on social media platforms, it seems nowadays everyone has some accounts so it's a big potential market.
I agree with you, about the solid and fair PoS coins. I prefer Peercoin though, because it has some nice developpment going on with Peershares.
Peercoin is a dead coin for sure.