Depends on your level of competence. If you run a safe operating system with a low chance of getting malware (ie, not Windows), and you make frequent encrypted backups, then the Bitcoin-QT client over Tor is the safest way to store your coins. The second safest place -- and I know this sounds crazy -- but the second safest place is probably MtGox. Say what you will about how shitty their service is, but they got hacked 2 years ago and have significantly increased their security with things like 2 factor authentication. They are the biggest target in the bitcoin community and haven't been hacked since, so I think that's a plus. I would trust my coins there more than anywhere else. The drawback is that you can't access your account over Tor. Next is blockchain.info. They also offer 2 factor authentication, and you can make automated backups of your wallet. My concern is that their interface has a history of being buggy and I've seen a few threads where people complain about transactions that haven't gone through. After that, it's really a crap shoot with the various ewallets. Some disappear over night, some get hacked, some have accidentally deleted their wallets.