Decentralized & open sourced one's.
For me, the best alternative coin for altcoin exchange is from Ethereum to Bitcoin. Why? Because Ethereum and Bitcoin has the highest exchange rate than any altcoin, I guess. When you have tokens, you have ro convert it to Ethereum for you to convert it to Bitcoin. There are charges in every conversion so just be aware. Choose Eth to Bitcoin exchanger, it is the best.
You completely missed topic. It is not which coin is best, it is which exchange is the best.
I usually using Poloniex and Bittrex, I don't know I just feel comfort using it. But since read some comments here, I think I'm considering to try c-cex and Binance.
Poliniex is just fine for me, I don't like bittrex anymore because they are asking for personal informations and for unverified account withdrawal limit is too low. I've never tried binance, is it good exchange?