I mainly use two sites and I work with these sites for about one year.
1. Coinbase
2. Cex.io

Coinbase has low fees than cex.io but it does not support the majority of countries. cex.io support almost all county but fees and prices of the currencies(bitcoin, Eth etc.) the little bit high than Coinbase.  Now Bittrex is supporting fiat trading. I think it will be good but I didn't try yet.