It's always been and always will be Granny Weatherwax for me.
Others that stand out:
CMOT Dibbler - in whatever guise or distant family relative he appeared
Gaspode - er Woof?
Corporal Nobbs - cos he kicks people inna fork.
Archchancellor Ridcully - its them early morning brisk runs and his need to shoot anything that flies, flaps, runs or skitters past him
Bill Door - Skinny bloke with an odd smile
Reg Shoe - only because he reminds me of someone I know quite well!
Theres so many memorable characters - I've always enjoyed them much more than the plot!