Honestly? My knives and chopping boards, with the Breville press a very close second.

I live for toasted ciabatta sarnies.
Seriously though, good knives and boards underpin good meals and a good cooking experience for me. I hate cooking at other people's homes when they don't have adequate stuff. I nearly came a cropper trying to cut a swede with a bread knife on a marble chopping board at my friend's house - I'm surprised these people manage to keep their digits!