Which AltCoins To Buy?

I am constantly hearing lots of people saying they were going to buy Bitcoin, while the others and more knowledgeable were already planning on getting into AltCoins like Litecoin, Dogecoin, Vertcoin, Siacoin or whatever the cryptocurrency-of-the-week was - but most people never took the plunge and actually bought some.

Now the question that I have, which affordable Altcoins that is promising high returns in a long run - next 3 to 5 years?