If I remember correctly, MtGox was one of the most trusted exchanges.

That's the problem with trust.  By the time it is discovered that an entity shouldn't be trusted, it's too late.  Many people have already all told each other that they trust the entity, and you quickly regret listening to those people and you wish you hadn't trusted the entity in the first place.

The problem with bitcoin exchanges are that they are always advertised as stable, secure and impenetrable. But we are reminded by various events show us that this is bullshit.
MtGox once the biggest  exchange - thousands of BTC lost, BTC-e was once hacked, Cryptsy - hacked, GateCoin - recent security breach, bitcoins stolen.

If you can, don't keep your funds in any online services which is not under your direct control.

After the latest inside job of the ShapeShift 'hack' I would be very cautious with any Bitcoin exchange I am using. I keep hearing bad stuff about Coinbase as well and it is considered to be one of the most trusted and secure exchanges by many. Unless the Bitcoin exchanges are regulated and prosecuted by the government for any losses of customers' money they would be liable for in the event of a hack, I don't think we can blindly trust any exchange to hold all our coins because their word to keep our money safe is not backed up by anything. Then when the funds go missing in some 'hacker's' wallet you are left with nothing because Bitcoin is non-reversible.

I am only using bitfinex and poloneix most of the times and without problem but that depend upon how you gonna use exchange. I mean if you like to do bitcoin trading most of the times than bitfinex have lots of volume and for altcoin trading poloneix is good.

Kraken and Bitstamp are fine. The last one was hacked last year but there is always the advice : "dont leave coins on an exchange".

Well, there is Gemini, but who would trade there? This means 100% exposure. They are certified in NY, so yeah, pretty reliable USD/BTC/ETH, but they need you identity checked.

we all know that popular bitcoin exchange have mean that its exchange have best quality,because many people use and have good recomended,but honestly i can't mention about which best exchange for me,because i sell or buy bitcoin from people,not from exchange.

most of the bitcoins exchanges declared that they are stable,trusted and very secured
for them to get trust of the community but some of them are not true so we must not be a victim of it.
for now one the trusted exchanges for me is yobit.net or if you want to know other exchanges
you can check bitcoins exchanges review.

I wouldn't to defined which the best bitcoin exchange site. because many people it always has a problem with exchanges like too difficult to withdrawal and another reason. Maybe :ccex,bitrex, or yobit.

Agree, one solution to this is never to store too many funds FIAT/BTC on exchange.
Avoid that at all costs, because if u don't it will be your cost

I heard bitfinex doing good, im using bitstamp coz i like their GUI mostly.