My all time favorite is the HP32S. It has a lot in common with a
whole bunch of other HP RPN scientific calculators, but it hits the
perfect balance between features and keyboard clutter. It also has
one letter program prompts that are very handy to remind you what bit
of info you are expected to supply to a program.

For instance, I calculate my work day in tenths of hours with a
program I call up by hitting XEQ T (for time). It asks S? I input my
start time (with a decimal point between hours and minutes o'clock).
I hit RS (run stop) and it asks Q? and I enter my quitting time. It
asks L? and I enter the duration of my lunch break. And it replys
with H=hours.tenths of hours. These simple alphabetical prompts make
it a lot easier to remember what an almost forgotten program wants and
supplies, without being a chore to program or needing a small
character display.