I really only know Gnus; it’s been a decade or two since I used anything else. I am also not really sure how to judge user–friendliness in this case. Now that I have customized it to my liking, Gnus displays exactly the information I want and has exactly the operations I want bound to keys that I like. That makes it very friendly to me, the only user I _really_ care about.
I think the design goals of Gnus were not about allowing the user to get started quickly by having the same buttons as every other email client, but instead empowering the users to extend Gnus so that it does what they want. As a result, it may take longer to learn how to use Gnus but the ceiling is much, much higher.
However, for bulk sorting of email I should actually have mentioned Sieve. It’s a standardized configuration language you can use to tell your server how to sort your mail, so that it is already done before your email client even logs in. Most IMAP servers support it, and it is pretty easy for email provide access to it.
You can write simple rules like this:
if address :is "From" "bugzilla-daemon@mozilla.org"
{
fileinto :create "INBOX.Bugzilla";
}
elsif header :contains "list-id" "emacs-devel.gnu.org"
{
fileinto :create "INBOX.emacs-devel";
}
and so on until all of your mail is sorted how you want it. It’s not a full programming language, so it cannot do everything, but it’s quite capable and pretty well standardized so you can take your filter rules with you if you change email providers. I use it even for simple rules like this so that all I need to do is paste them in when I move to a new provider. One doesn’t need to do that very often, but it only takes seconds to do instead of minutes or hours to reenter every filtering rule in a new UI.
My current email provider is Fastmail. I discovered after I signed up that most of their mail–filtering settings get turned into Sieve rules that are shown to you when you go to add your own rules. I really like this because you can see exactly how spam is handled, how your vacation responders are implemented, how email from blocked senders is discarded, etc, etc.