My favourite is Poloniex.
This might sound funny but I also really like yobit's layout and design, it all seems to so... natural. Their support and everything else sucks though.
My least favourite is Bittrex, they might be reputable and "safe" but I hate their layout with a passion, I will never get used to it.
another question about the design which you like light or dark?
Well, If I had too choose I would go for dark but the actual layout of the exchange is more important to me.