I need an E-mail Client

I often argue with my collegues about e-mail client applications. I've used the Thuderbird for several years. But I don't recommend it for anyone whom I have no will to support. It's still been troublesome for people who takes the e-mail system for granted.
There're so many alternatives but the best one. I wonder which e-mail client dose smart people use?

The mere features I need for the e-mail client are all related with treatment of received e-mails. That is, clever spam filter, incremental search within e-mail headers, and to categolize e-mails based on keywords or search results. I don't stress the feature for writing e-mails after all.