This is the top 7 list if we are talking about fair (Darkcoin excluded) and really decentralized currencies
The ray of truth - Select Filter non mineable and premined
I am not sure about Dark if it is POS or what but PPC is non mineable i think. So, excluding Darkcoin(Instamine), Peercoin(POS). Doge and Namecoin dont add to a needed and empty space.
We are left with three coins only then, Bitcoin, Litecoin, Monero. If Litecoin loses its steam we have two currencies with different uses complementing each other