The absolute "best" filter/conditioner for digital

I don't know about "absolute," but the best I have used is the Pure Power + (for either digital or analog).

Foundation Research LC-1

The difficult thing about power products is that, while 2 people can have the exact same system, they don't have the same electricity. What works for one person may not work for someone else. If you are looking to buy something, I would call The Cable Company. They will send you anything you want to try so you don't get stuck with something that doesn't work for you. I've been using them for years and wouldn't consider doing it any other way.

Zd542's advice is spot on, but The cable company doesn't carry everything out there. Because of the unique interaction of your electricity with your system, no one can tell you what the best will be, and you could easily blow your whole 401K balance running the experiment. Sadly, finding the best is like winning the lottery, but finding something that works really well is not so hard. In addition to the Cable Company, talk to Chris at VH audio. The guy is a real straight shooter.

I came late to the power treatment party and have been astonished at what it offers.