The simplest one to read may be BCD (binary-coded-decimal) displays. They can be horizontally oriented like here:
Then there is a pure binary display, to be read horizontally like here:
As a vertical binary display takes five lines, it cannot be shown on this display.
It is not trivial to read these display, so every now and then the display will show the time and some additional help information, like here for the BCD horizontal display:
or here for the binary display:
Finally, here is a display in the form of horizontal bars:
It shows hours in a 12-hour format, so the time shown is really 04:33:38. The horizontal bars are grouped in four times three squares, corresponding to the four quarters of a round clock face. Then I use symbols with 1 stroke, 2, 3, and 4 strokes to denote intermediate values. Thus the lower line shows (3+3+1) * 5 + 3 = 38 seconds.
Neither of the digital displays are really very straight-forward to read. Which one do you like? and are there other variants of similar displays that could have been included?