By far the best email client I used on any computer was Sparrow for Mac. However Sparrow is not a cheap app, some will find it difficult justifying the £6.99 price tag when they can easily get by with the standard interface for services like Gmail and Windows Hotmail.
Inky is an app I've just come across which is the next best thing. Best of all it's free and available for both PC and Mac. The first thing that drew me in was the flat and gorgeous design which Inky rocks, no ridiculous over the top designing, a simple email client to make your email simple.
With email becoming such a mess, especially if you have many accounts, it can be difficult to keep track. Inky allows you to add all your accounts into one. It supports nearly every type of account out there.
It includes a bunch of useful features to make email a lot easier, such as sorting and filtering, email syncing and so on. There aren't no special amazing features which have never been seen before. Inky's purpose is to allow you to handle all your emails from one place.