Hands down,
Firm Express. Originally bought full price, then used so many times I wore out some of the disks, and repurchased super-cheap from Amazon. I'm doing another FX rotation right now!
My other best investments:
Brazil Butt Lift. I only use Bum Bum (and occasionally Cardio Axe), but that one workout alone was seriously worth the price of the whole set.
Amy Dixon's Ultimate Fat Burn--favorite workout ever ever ever.
Cindy Whitmarsh Ten Days to a Better Body--second favorite workouts ever!
Denise Austin Fat Burning Blast: Used so much I wore out the VHS tape and had my dad put it on DVD for me. This is the workout I *always* come back to when I'm re-starting or need a quick pick-me-up. It's pretty much perfect.
...And I'm adding
Classical Stretch Season 10 to the list, even though I've only done a couple of them (from the discs) and they were pricey. But they are there when I need them, and I am so glad to have them!!