Definitely QRK.
It is from a technical point of view the best developed coin: fast, secure and good financial structure.
Besides there is a great development team behind it trying to integrate QRK within the community. There is a lot in development: mobile wallet, pay systems, website widgets etc. Just have a look at reddit/r/QuarkCoin or at the Qaurk foundation forum and see how active they are
At exchange sites it is one of the most traded coins, and sites selling QRK for fiat have troubles with a good supply of the coin.
The price is ridiculous low for such a potential coin. In my opinion this is an investment with low risk and you will have lots of fun with QRK on the long term!
Unfortunately the best quality coin has not always the best price. Marketing is another important aspect of gaining value. One coin is master in that: DOGE. It has much sympathy from the community. In my opinion it couldn't go higher than 150 satoshi. This joke reached this week almost to 300 satoshi. It will end somewhere but where I have no idea. Probably at the moment BTC will find its way up again. The problem with their amount of coins circulating, lot of mining rewards and high price is that it will fall hard when it starts to fall. A high risk coin to invest at the moment. Just wait till it is below 100-150 satoshi again is my advise. At a higher price I would not invest.
LTC is a steady coin. On the long term (> 1year) it will gain some value again. You will not loose any money by investing in this coin now to my opinion. It is like QRK an unique coin with lots of benefits compared to BTC. Most coins are cloned LTC (including DOGE), what is more promotional than that?!
Other coins I have not so much experience with. PPC has a great developer (the same as XPM), but what I heard is that the community is not that active. At the end you want to use your coin for payment transactions, otherwise the value of a coin is of no use. Because there are big whales behind it, you will not run in much risk investing in PPC. When the price is too low it will be pumped some day. That is my experience with investments in this coin at btc-e. Just do not invest after the pump
, or while it is pumped. I lost some during those pumps with PPC and XPM at btc-e.