Blockchain.info in theory should be quite secure. They offer pretty good security features, allowing you and only you to be able to access the private key thus no one else can spend it. However, they have incredibly bad reliability due to their recent downtimes and several security vulnerabilities which could have been avoided. Greenaddress.it is quite a good choice but the android app is slightly buggy for me. I would recommend either Multibit or electrum for the best ease of use as you can easily forget your mnemorics on greenaddress.it.