I'd not heard of the 1951 match (that Ivan's referred to), but as I've only spent time around blue Sheffield - and that game was a helluva long time ago - I guess it's not surprising. Based on my time hanging around south Yorkshire (and on the once brilliant OwlsAlive message board [RIP]), I'd say it's a tossup between the Wembley semi and the Boxing Day Massacre. Both are still referred to regularly by those old enough to have been there - you hear about them all the time. In more recent times, the Owls 2-1 win at the Sty in '09 was huge. It was the first League double over them in 95 years (which I still find shocking). I've seen Wednesday beat them a few times since I started going over; but the reaction from Wednesdayites was for this one far more intense than any of the other wins. They had a 'beamback' at Hillsborough, where they put up a huge screen in front of the south stand, so folks who couldn't get away tickets could watch the game with their fellow fans. Have a look at Marcus Tudgay's winner: I'll also toss in a 1-0 win in February of '12 - though it's magnitude was only realized in retrospect. With United well clear in the table and apparently cruising to promotion (from League One) the Derby win (after which manager and Wednesday legend Gary Megson was inexplicably sacked) kicked off an11-3-0 run to close the season, pipping United to second place, and dropping them into the playoffs, the final of which they lost on penalties. United had been, I think, eight points clear with a game or two in hand (or something like that - don't quote me) that Spring, but Wednesday began clawing them back in with that derby win in late February. But in the end it's gonna be either Wembley or Boxing Day - take your pick.