BTC-E is the best. Here are the reasons:
1. One of the oldest exchanges
2. Located outside the EU / US
3. Ease of deposits / withdrawals
4. Availability of other crypto-currencies
5. Right now, the largest BTC volumes.
6. Never been hacked or DDoSed.
+1 for BTC-E, been using them for a long time and they have never let me down.