I am a real spice fanatic. I think they can change an average dish into something truly remarkable. I used to keep all my spices on my windowsill but learnt that this was ruining their flavour. I still decant all my spices into labelled jars but now they are kept safely tucked away! I use old jam jars because they are a good size but you can use any small jars you like or just keep them in the original packaging. Here are my spice essentials. You will see that some of them overlap. I would also recommend investing in a marble mortar & pestle to grind your spices in.
I tend to buy my spices from Spice Mountain in Borough Market or the Spice Shop in Notting Hill. Both have a vast array of spices on offer and if you can't get to the actual shops, both have online stores.
Mild, medium & hot curry powders
Yellow & black mustard seeds
Za'atar (I get mine from Israel & I would suggest you go to a middle eastern supermarket for this)
Shwarma Spice (I use the Pereg brand)
Cardamom Seeds- green & black
Chinese 5 spice powder
Kaffir Lime Leaves
Maldon salt/flaked sea salt
Not essential but nice to have anyway:
Dried Rose Petals