Edit: I'm going to have to retract my comments about cheapest price being on official hotel brand websites. I did a cursory search and find many cheaper options on Hotel Tonight (and expedia) for same day reservations than the brands, and while I haven't looked at all the brand's best price guarantees, at least IHG's exempts them from having to provide the lowest price within 24 hours of checkin, so it seems like a loophole to let them dump rooms for cheaper on third party websites day of arrival.
The Best Price Guarantee is largely a lie. It's a Best Price Guarantee on a rack-rate rooms. The problem is most OTAs will offer a significant discount (sometimes more than 50%) for a non-refundable room. The rooms are exactly the same, they just classify them differently, so they don't have to actually have the best price.