Im using different exchange depends where altcoin I want to trade is available. Oftenly Im using bittrex, polo and hitbtc. Sometimes Im also using etherdelta to exchange erc20 etherium token specially if its just few tokens.
Polo has serious problems, check my previous posts please. Don't get scammed by poloniex.
To answer the question, I actully tried all the exchange platforms. Bitrex, liqui, hitbtc are trusted ones without deposit/withdraw problems. Etherdelta is also working smoothly but the volume is low. I tried Cryptopia, it's also good without problem, but volume is still low in there too.
I am also using Polo and I have some of my coins in there. I are frequently traget by DDOS and logging in them was very slow. Also I am using Bittrex and Liqui and sometimes kraken but its quite expensive there.I used many coins depending on the coin where it is being traded.