Btc-e has the best liquidity, the server does a fairly good job against ddos and prices update quickly / data is fairly reliable. I haven't had any issues.
Vircurex just removed support for two coins which I'm disappointed with despite their reasons. They haven't put back the withdraw buttons on some currencies yet either and they have some technical problems with reliable data. You have to "release" trades and withdraws as well which I find annoying as hell.
I tried to use
https://cryptonit.net/exchange/1/2, it was a bit too slow for me but it looks promising.
