There's an interesting discussion of similar matters on a competing Minnesota radio board at the moment, and some folks over there have named 540 CBK-Watrous, SK (Regina market) as the most likely candidate for maximum DAYTIME coverage. It has the mid-continent's amazingly good ground conductivity in its favor, as well as its low frequency, PLUS the CRTC lets even low-band stations run 50 kW. (Compare 550 KFYR-Bismarck and 570 WNAX-Yankton, which the FCC only allows to run 5 kW.) Check out the Radio-Locator map for this signal -- even the 0.5 mV/m (purple) contour goes well past the map boundary!
But as far as NIGHTTIME land coverage, I'd probably have to concur with the person who mentioned WBAP and WOAI. Both of these are regular nighttime catches here in St. Paul -- despite WBAP's adjacency to 50 kW local WCCO -- combine that with Joe's mention of reception in Las Vegas AND Ohio, and you've got a pretty good candidate. (One thing, though -- isn't skywave propagation notoriously unpredictable in comparison with groundwave propagation? Wouldn't that make it difficult to name a sure winner?)