EDIT: This first paragraph previously only referenced the Harrier, but I've since found evidence of the Osprey doing the same, and the F-35B has since been reported to perform the feat.
The only true answers I'm aware of are the Hawker Harrier (or Harrier derivatives thereof), the Osprey, and the F-35B. These are all VTOL aircraft which can fly backwards, and can also fly very fast forward... thus they are the "fastest" aircraft which can also fly backwards.
Previously this crown belonged to the Harrier, but since the F-35B is faster it now takes the accolade of being the "fastest recorded backward flying plane": it can fly backwards, but also fly at mach 1.6.
As to your original question, the answer is that any aircraft is capable of this feat of "moving in reverse relative to the ground", if the wind speed is higher than that aircraft's stall speed.
I'm not quite sure what your question is, as you ask what is the fastest plane which can do this: Do you mean "What is the maximum speed of any aircraft which can achieve this?" or did you mean "How high can this relative speed be"
For the former: With the maximum wind speed recorded on Earth being over 230mph, any aircraft with a stall speed of 230mph or lower (ie, almost any plane), can do it. Therefore the fastest aircraft which can achieve this is the fastest aircraft on the planet. I'm not 100% sure on the fastest aircraft, but the SR-71, for example, can fly at well over Mach 3, but is also capable of flying "backwards" if the wind speed is above around 180 knots.
In theory the SR-71 could therefore fly at Mach 3 "forwards" or around 50mph "backwards" (in the context of this question) by travelling at 180 knots into a 230mph headwind. I wouldn't recommend attempting this, however....
Or to answer the real question I think you're asking which is "How fast can an aircraft performing this feat travel relative to the ground", the answer would be "the aircraft with the slowest possible stall speed" and the relative speed would be approximately 200mph.
An aircraft which can fly at under 30 knots, flying into that 230mph headwind, would be travelling "backwards" at 200mph relative to the ground, and at 30 knots relative to the wind.
So actually with it's very low stall speed the An-2 is probably a contender, although I believe there are aircraft with lower stall speeds.
(Apologies for mixing Knots and MPH almost at random in this reply: all numbers are approximate, however, so it probably doesn't matter too much. For the purposes of this answer, I'm treating 1 kt and 1 mph as approximately equal)