You have three options to answer this question...

1) Do some research yourself, read the forum threads over the last several days to see what is popular/trending/etc.

2) Use sites like coinwarz.com or coinchoose.com to gauge profitability - both sites compare current difficulty and exchange prices to determine what is most profitable at this very second, although neither site is perfect.

3) Use a pool that makes the profitability decisions for you and pays out on BTC daily.  I have used http://hashco.ws and http://middlecoin.com and I like both of them.

I would recommend #3 until you get comfortable and #1/#2 as a longer term strategy.