Hi
I know donuts are not varieties but are interesting never the less.
I was looking through my cancelled stamps and came across my Scott 981 stamps...I had 94 of them and found 89 had donuts..wow
Here are some examples that I found..To my knowledge there are 6 different style donuts as shown below.

Here are 3 with a circle donut within a circle donut..Also circle donut with no inner circle...And half donut with over lapping printing.

Here is on the left what I call "half donut"...Next is a full donut with solid colour like the right one also.

Left one overlaps 2 colours on the stamp...Middle one is half hidden donut but with a center donut...The right one is a little rarer...It is an oblong donut.
IF ANYONE CAN FIND A DIFFERENT DONUT THAT MY 6 TYPES, PLEASE POST.
Robert