As of this moment, Diana Byrne's count of 30 Black Terns in Harney County comes up at the top of the list, followed by a few recent twitches of single rarities (Hooded Warbler etc.). Doug Robinson's remarkable estimates of warbler numbers from April 25th show up on this list, though at this point they're buried under more recent highest-of-year counts for other species (including a report of 7 Great White Pelicans!!!).
I suppose one could check this list daily for recent high counts, though you might have to winnow through a lot of chaff before picking up on any extraordinary movements. That seems like a chore to me. Perhaps someone else knows of a more efficient way to keep tabs.