Mario Cipollini: 1943 watts. Although that's probably the max reading from possibly a minute long sprint, depends on the course. There's probably slightly uphill sprints that would have even more max wattage potential since top pro sprinters on flats are probably maxing out what they can neuromuscularly coordinate on a 53x11.

But anyway, if you know who that is, you'll know that 2000 watts is not a common max output.

Edit: Ok I was a 320 watts for 20 minutes guy a few years ago, I'm probably about 250 watts for 20 minutes guy now. I just did this workout, probably in the 500-600 watt range for the 20 second surges, and it was surprisingly hard.