The best candidates so far are primecoin, ppcoin and namecoin. Of course there might be better speculation opportunities around but those 3 look pretty solid so far.
Please don't spread bullshit.
Primecoin's only purpose is something that frankly outside the scientific/math departments, no one else cares about. It simply finds prime numbers, big whup.
Peercoin may be the best best out of those 3 you listed, but it's dead with a shriveling community, and would probably never reach as high as litecoin did.
Namecoin again is a failure. It's decentralized Domain name feature will never work, decentalization does not work with everything. For EX: There are people squatting domain names like microsoft.bit, apple.bit, and theres no way to enforce copyrights if you own a name(like microsoft etc). Basically, namecoin is controlled by squatters and theres nothing we can do about it. Stick to ICANN.