Cinnamon is probably my favourite spice. I love the flavour, and it works so well across both sweet and savoury dishes. Plus it is great for regulating blood sugar, lowering LDL, cholesterol and triglycerides, is anti-viral and anti-microbial (so great if you are going sugar free due to candida overgrowth.) I try to have about 1 tsp a day, so in a smoothie, in tea or coffee, on fruit or with coconut milk/cream.
I also have a fairly large collection of other spices (and herbs) which are so useful for huge flavour without the calories, additives and plenty of benefits unique to each spice. I buy Spice & Co ones from Harris Farm and they stay super fresh in their little zip lock bags. Some of my most used are cumin, paprika, chili, tumeric, ginger heaps of herbs like basil, sage, oregano, rosemary.