My favorite has to be the d%, otherwise known as the simplified d100.
As a DM, it helps me with a LOT of things, from rolling on randomized tables to creating tables to use. Very nice for making random NPC decisions too. Lol.
The d20. So many sides. *Stares with wonder at a d20.*
I am fond of the d10. It's a nice, solid die for health OR damage.
Tie between d12 and d20.
As a DM/GM the brain is probably my favorite die (i.e: Whatever I want the number to be).
As a player it'd be the D8 I always have such amazing luck with those things. D20s on the other hand... *fist shake*
I'm glade this group hasn't completely died yet.
A new appreciation I hold, though, now that I'm a player character again has got to be for the d6. Because nothing feels quite the same as grabbing a fist full of d6's and throwing them down for damage.
I only get a single d8 for damage, sure I get +1 but I'll have to give the enchanted bow back at some point...though that's after surviving being stranded on an island infested by the undead.