Outlook as part of the Microsoft Office 365 package would be my recommendation. Not only does the program work extremely well the single .pst file makes it incredibly easy to backup and restore your email and contacts in the event of a failure.
Thunderbird is an excellent free program and has also served me well for many years.
Interestingly enough, many of my clients have used IncredImail however it does have a bad habit of leaking data. By all reports it works rather well however I wouldn't think much would be private or secure given the way it works. Maybe things have changed over the years.