We've been using free/ make your own curriculm around a monthly theme. There's already a lot of good resources online readily available, so my friends and I didn't see the point in buying one when we came up with our system.
Although we are taking a break right now. If I were to buy one right now it would be Waldorf Essentials.
I prefer to make your own preschool curriculum. You can get a lot of ideas online about this. But purchasing pre-package preschool curriculums can definitely reduce your efforts and time too. But I think preschool curriculums are very effective in preschools where they learn in groups and enjoy.