So many of these threads recently. It's also somewhat dependent on which country you're from. Just a very quick summary of the top exchanges:
mtgox is the largest, easy to get bitcoins in, but it's not possible to withdraw money out once you've sold your BTC if you're in the US. Other countries should be ok, but you should check first. Although, they have strict verification process that could take a while after you upload documents of proof.
Do NOT use
BTC-e, once you deposit money into your account, they will indefinitely hold it and not credit your account. A lot of people are having problems taking money out as well.
Bitstamp is reliable for the most part, but you need government ID and proof of residency verification before you can start using them, which could take a few weeks to verify.
There's also
localbitcoins.com that seems the most reliable and the quickest to get BTC or money. The most popular payment method from localbitcoins is in-person cash transaction, or bank deposit directly to the seller. There is automatic escrow built it if you don't feel safe.
