I personally use cex.io, localbitcoins и @BTC_CHANGE_BOT in telegram. Cryptonator as well, sometimes. You always need to compare the prices. Do not forget to include the commission.

For example, on site#1 you can see $4000=1BTC and 5%comission.
on site#2 you can see $4100 and 0% commission.
If you put 4100 into #1, you'll end up with .97375 BTC and on #2 with exactly 1. So always compare total rate including the commission.