Decentralized Exchangers are the safest trade areas in my opinion. There's a lot of scams in this market. Be sure to check every process over and over again. What is the most secure cryptcurrency. I think it's bitcoin. Bitcoin volume is the largest.
The problem is that with decentralized exchanges it's almost impossible to use actual fiat to trade. This is because the actual decentralized trading only exists on contracts which is possible only with ethereum, which means that you can only trade the tokens with bitcoin. The closest thing to decentralization with btc and cash that you can get is bisq, however, the liquidity of it is pretty much a joke.