Despite all the other answers, I suspect Web Browser is the most popular. As web apps for email got better, development of desktop clients stalled.
Fast search through a lot mail takes some considerable resources to build, store and search an index, and web-based systems do that really well.
I’ve used about all of them over the years: Pine, Mutt, Thunderbird, Evolution, K-Mail and some others.
I eventually threw in the towel and use web UIs now. Fast, available everywhere and good keyboard support, especially when paired with a browser extension like Vimium.