You need low diff coins that the USB miner supports.

There's two kinds of USB miners today: SHA-256 or Scrypt.

For SHA-256, use this list to find the most profitable:

http://whattomine.com/asic?utf8=%E2%9C%93&sha256f=true&factor%5Bsha256_hr%5D=16&factor%5Bsha256_p%5D=4&factor%5Bscrypt_hash_rate%5D=500.0&factor%5Bscrypt_power%5D=800.0&factor%5Bx11_hr%5D=15000.0&factor%5Bx11_p%5D=1200.0&factor%5Bqk_hr%5D=450.0&factor%5Bqk_p%5D=75.0&factor%5Bqb_hr%5D=450.0&factor%5Bqb_p%5D=84.0&factor%5Bcost%5D=0.13&sort=Revenue&volume=0&revenue=24h&factor%5Bexchanges%5D%5B%5D=&factor%5Bexchanges%5D%5B%5D=bittrex&factor%5Bexchanges%5D%5B%5D=bleutrade&factor%5Bexchanges%5D%5B%5D=cryptopia&factor%5Bexchanges%5D%5B%5D=hitbtc&factor%5Bexchanges%5D%5B%5D=poloniex&factor%5Bexchanges%5D%5B%5D=yobit&dataset=Main&commit=Calculate

For Scrypt, use this:

http://whattomine.com/asic?utf8=%E2%9C%93&factor%5Bsha256_hr%5D=16.0&factor%5Bsha256_p%5D=4.0&scryptf=true&factor%5Bscrypt_hash_rate%5D=4&factor%5Bscrypt_power%5D=4&factor%5Bx11_hr%5D=15000.0&factor%5Bx11_p%5D=1200.0&factor%5Bqk_hr%5D=450.0&factor%5Bqk_p%5D=75.0&factor%5Bqb_hr%5D=450.0&factor%5Bqb_p%5D=84.0&factor%5Bcost%5D=0.13&sort=Revenue&volume=0&revenue=24h&factor%5Bexchanges%5D%5B%5D=&factor%5Bexchanges%5D%5B%5D=bittrex&factor%5Bexchanges%5D%5B%5D=bleutrade&factor%5Bexchanges%5D%5B%5D=cryptopia&factor%5Bexchanges%5D%5B%5D=hitbtc&factor%5Bexchanges%5D%5B%5D=poloniex&factor%5Bexchanges%5D%5B%5D=yobit&dataset=Main&commit=Calculate


Note that all I did is use the filters on whattomine.com for the hashrate of the common SHA-256 (15 ghz on a GekkoScience 2Pac) and Scrypt (4 mh/s on a Futurebit Moonlander2)