It all depends on your personal preferences. My most important criterion is safety. I want to feel that my coins are safe so I use only reliable exchanges. Moreover, fees are pretty high on some platforms, it's good to find the exchange which prices are reasonable. What's more, I buy not only BTC but also some altcoins like XRP or ETH so I need the platform which broad variety of markets. Now, I use CoinDeal and I can say that this is the platform which definitely satisfy me.