I don't like to spam with links, but http://www.bitalo.com aims to be the service you described, and will be launched soon. "Most secure" really means that no one, even the site admins/hosting platform can never touch your coins. This will be enforced by the technology used, not just some internal policies.
And also, the site will be backed by a german AG company, which is basically a type of "Public limited company" backed by minimum of 50,000 EUR.
I plan on using Javascript crypto. I've spent many hours reasoning over this idea, also studying security community reactions on mega.co.nz and I think it is possible to do now. I don't want to disclose all details now for obvious reasons, but all I can tell that I will not reinvent the wheel here - all parts needed are already available and mature, you just have to assemble them into a complete solution.
The point is that you are overstating the security benefit of your solution. A breach of the server will now only compromise wallets of anyone who logs in until the breach is detected instead of all wallets instantly.
Blockchain.info can't touch my coins. But if there servers are compromised, a hacker could inject a tiny, tiny amount of JS and have my ID/password sent to... anywhere... and then the hacker could access my account. I'm curious to know how you'll get around that vulnerability.
The security on the client side will be on the level that blockchain.info provides, but this will be more than just a online wallet - it will also be an exchange.
I'm aware about injecting JS vulnerability. Of course you can't get around it with anything on the main server. It's possible however to setup an external server that will be monitoring the files and firing alarms the minute something's wrong (asset checksums doesn't match). And that's exactly what we'll do. Another thing is that all SSH/SCP access is also logged and the whole team gets an email immediately when it happens.
