What is the most efficient and user friendly email service?
If you want a web-based Inbox - go for Gmail.
If you need a desktop email client to track several email accounts - try out Mozilla Thunderbird or EmailTray. Thunderbird offers lots of functionality but it loads your PC because of all its bells and whistles. EmailTray is lightweight, and it offers enough features to manage all your email accounts efficiently. Both Thunderbird and EmailTray are free of charge.
I find that Gmail does everything that I want. It has plenty of space and does everything that you need from an email account. Have been using mine since it was in beta.
The most efficient and user friendly email service is Gmail. It has POP/IMAP features using which you can download all messages in your favourite desktop email client like Thunderbird or MS Outlook.
Your question has two answers. My favorite Desktop email client is Thunderbird but if you want a web based email service I would recommend Gmail which in my opinion is the best email service around.