Well, it depends if you would rather use your coins right now or in the future.
If you want right now, then Litecoin is the way to go. Generally it rises and falls in line with Bitcoin's value and it's accepted in more places than any other alt.
However, if you want to speculate and look to the future, I'd recommend looking at PPcoin or Freicoin.
PPcoin introduces Proof of Stake as a way to mint coins. It works similarly to interest, after you've had coins in your wallet for 30 days, you can earn 1% of your total coins every year i.e. if you have 100 PPC you get 1 PPC every year + 30 days.
Freicoin is the opposite, it has inbuilt demurrage which means that it depreciates in value (5% per year).
This provides a deterrent to hoarding and encourages spending and better free market trade according to the website.