I started on the Psion 3a, then 5mx, Nokia E90, Nokia N900.
And then sadly I ran out of good options.
Nothing beats a good keyboard (though it does also need to be a good one).
I would love another clamshell design, maybe with an e-ink display, running debian like my N900.
EDIT: I do have a beady eye on Planet Computing's machines: https://www.www3.planetcom.co.uk/devices