Xapo is an intuitive and secure web bitcoin wallet.
It operates via a majority of the web browsers.
For purposes of storing the private keys, Xapo uses offline servers that are never connected to the internet, which makes its system security unbreakable.
Backed by Silicone’s Valley creme de la creme, Xapo has a wide client base, too.
The only criticism seems to be their weak support team.