Don't use a centralized exchange. Don't use a hosted online wallet. Any centralized source of a large amount of BTC/XBT or any other currency thereof is simply going to be a target, no matter what. Anybody that claims they are 100% secure should be put into question. 0days are very real.
If you would like to use an "online" wallet, use Blockchain.info.
If you would like to use an "online" wallet, use Blockchain.info.