I've heard a lot bad experiences of Bittrex & Poloniex, is there a reliable platform recommend from your own experience?
Bittrex and poloniex don't even deal with USD.
They do use USDT but then you have to convert your Tether into USD by either getting verified on the actual tether.to website or use an independent third party exchanger. I don't think this is what you want.
I would recommend either use Gemini, Bitstamp, or coinbase if you are looking for simplicity. However just keep in mind that coinbase is currently having some support issues, would not recommend you use them just yet.