Coriander seeds
The most useful spice to have in your kitchen as it mixes well with any combination of spices, sweet or savoury.
Ground coriander seed acts as a thickener in curries etc. Use fresh seeds in sweet cooking, but roast them before grinding for savoury dishes as roasting brings out the complex flavours. Coriander can be used to balance any overpowering spices in any dish or mix. It is almost impossible to use too much coriander.