Which client is the best for productivity?

It all depends on what you do with your emails, and how many Email Accounts or Addresses you have.

I personally have found that Gmail works best for me.
I have several different Gmail addresses each one is used for different things. It all work on my phone, tablet, and PC web interface works great.

Some people think i am crazy when I tell them this, but it works for me.

Breakdown of account uses. (some people think I am crazy, but it works for me)

one is for friends and family

one is for places I do business with

one is for Online forums

one is for Banking

one is for Medical stuff

one is for US Government dealings

one is for Entertainment resources.

This actually sounds good to me. I'm about to fully switch to Gmail (so far for work I used Outlook, can't say I'll miss it) and I am not sure how to organize stuff. Not to go into details, but there might be some overlapping in my case. Still, I might have to do something like this. It feels like Tetris honestly or a loaded container. I can't stop visualizing my Gmail that way - stuffed top to bottom, things need to fit into the right place, or else...