If you want a large return within 30 days, I'd go with Steem.
If you're looking for something to day-trade, I'd say BTG right now because it's been showing moves of 15% each day. I can say from personal experience, that several trades of +10% profit really start adding up.

Why steem? It's been going up and down in waves the past half year. We are currently very close to it's low and its due for a shot up towards 1.60. Even if it only hits 1.30, you already got over 30% return on your money.

Bitcoin is the most unpredictable right now because it could go either way. With a lot of BFX cryptocurrencies most likely going to follow the downfall of BTC, you'd be pretty safe in STEEM, and possibly even in BTG. (Assuming BTG shows an inverse relationship with BTC)