Bitstamp is a cryptocurrency exchange that is located in Luxembourg, and it is by far the largest exchange in terms of trading volume and trading pairs. It was launched in 2011.
In terms of regulation, it is the first exchange to get full license in the world. This has made it to be very strict in all ramifications. Although intending users can sign up freely, rigorous KYC is observed before deposit or withdrawal. I appreciate this because on some exchanges, deposit will be accepted without regulations whereas they will insist on regulations for withdrawal. This has happened to me personally.
They also score an 'A' in terms of security. Since attempt was made to hack their account years ago, they have put in place very high security system that can surmount any attempt to hack them.
Another point is that the exchange offer cryptocurrency to flat exchange. Purchases can be made with two major Currencies i.e. EUR and USD.
It is also transparent. It's accounts are regularly audited by four popular auditing firms, including delloitte.
To me personally, Bitstamp is an exchange with classic reputation. Anyone who wishes to use a really reliable exchange should patronize them.