The best miner depends on what cryptocoin you want to mine.
So let's start: do you want to buy a hardware miner (ASIC), for Bitcoin-like coins or for Litecoin-like coins, do you want to mine with your GPU or CPU?

The profitability depends on how much you invest in the hardware, on the price you pay for electricity and on market, which fluctuates.

All in all, you have to give more data for better answers.