Like almost everyone else, the top of my list would be my rotary cutters and mats. In that same category would be my rotating mats - I love those and would have a hard time squaring up blocks without them.
Next would be my magnetic seam guides. I need a "dam" when I sew - a simple line or strip of tape just doesn't work for me; I end up with a wiggly wobbly seam.
The "big board" ironing boards that I made. I have two of them - one of them is 36"x72" and the other one is 26"x72". They've made ironing so much easier.
Last, but not least, my design wall.