Armory offline wallet is the best. Use it on a permanent offline PC and interact with a USB stick through it. Make sure the hard-disk is encrypted and also hide your passwords very well, its the best security that i can think of, secure enough to hold 1 billion $ woth of bitcoin in it.

But only if you know how to secure it propertly.

Otherwise for smaller sumps Xapo, Blockhain.info and other online wallets are pretty decent too.