Another happy Fastmail user here. They have great IMAP support (with app password) so I have a full local copy on my laptop that I can search instantly with notmuch or read/write from Emacs, but lately I've also started using their web UI a lot because it's just so fast and simple and well-thought-out. When they ship new features, it's stuff that makes you think "oh, that's actually a good idea – and they even made a very logical keyboard shortcut for it". And they've been very responsive to the few support requests I've made.