One my faves is Fastmail.
Paid (but inexpensive), formerly from Opera Software, recently bought out by the employees.
- A very complete IMAP implementation with UIDPLUS, ESEARCH, MOVE (and many more extensions I've never heard about)
- IMAP IDLE
- IMAP MODSEQ/CONDSTORE ("no change shortcut" in AquaMail)
- Domain hosting, family and business plans
- Nice web mail (no split view like in Gmail, but then I don't see them making drastic changes like Google does lately).
Disclaimer: I don't use it regularly, it's just a test account