No sense mining BTC with them; ASIC farms have pretty much made it impossible to find blocks with anything but specialized hardware. There are cryptocurrencies whose mining function is specifically designed to be ASIC-resistant and therefore which are still profitable to CPU mine. Monero [getmonero.org] is probably the most popular of them (and has the benefit of an encrypted blockchain, so others can't see the wallet to which your botnet is sending its ill-mined coins)