I'm not exactly herb and spice savvy, but if I had to choose one it would be cinnamon also. It smells so good. Sometimes I will just sprinkle some in a pan and let it simmer on the stove in the wintertime. Makes the whole house smell good. Sometimes I will do that with vanilla too.
Couldn't really say...though cinnamon would definitely be up there either in sweet or savoury dishes (grilled lamb on cinnamon (and other spices) and dried fruit couscous is just wow).
However recently I have been experimenting alot with the middle-eastern spice sumac (try it if you can, its quite unique...a recent brainwave I had was sumac flavoured ice-cream, I'll have to experiment with that, I wonder if there is a recipe for it somewhere).
Also because I've been delving a bit more into traditional Chinese cuisine for my Asian fix recently I have been reaching for szechuan peppercorns to throw in the pan then crush up and sprinkle over my dishes.
Cayenne pepper. I put it in everything from Mexican dishes to chocolate cakes/confections to...well...just about everything. It's one of two spices I'm seemingly always running out of in my kitchen (the other being black pepper)