This question is both natural but silly at the same time. People are just going to tell you the coins they hold or are biased towards. You're best off just doing your own homework.
https://bitcointalk.org/index.php?topic=134179.0 List of all coins.
http://coinmarketcap.com/ How much money is currently in each coin.
Most alt coins are not special, some of them do improve upon or add new features that BTC does not have.
Top 5 coins:
BTC - The King, the standard. Good choice for both short and medium term, but there are many talented people out there trying to take it out with a superior crypto-currency.
LTC - 2nd place coin. A lot of people are invested in it. I think it's a good choice for short possibly medium term, but I'm not sure if it will last long term due to lack of anything really setting it apart other than it's time start advantage.
PPC - 3rd place coin. Interesting natural inflation rate ( Moore's law ), original coin to add proof-of-stake, energy efficient, transaction fees are destroyed, ASIC friendly (unlike LTC). This is my biased coin
Join us at peercoin.net
Namecoin - I don't know how useful it's actual feature is, but since it's merge mined with BTC it might always follow BTC in a certain way.
Last coin changes daily, there are many battling for number 5. Primecoin, and Worldcoin are notable, Feathercoin is just LTCx4 but it has a strong following that just doesn't want to quit.
Safe bet? BTC, LTC, PPC, and 1 or 2 lesser known ones.
