It's a great argument, I am not sure.
I'd argue that last year's 20 point shellacking of DePaul in the opener was huge, because we unleashed a style of play unseen in the midwest by a DI team. It changed the whole philosophy of the program. Coming into that game, no one knew who would score points, and thought we'd be winning games in the 50's instead of 70's. It showed that BU wasn't going away, no matter what kind of team we had.
Last year's home last second win over SIU was pretty big as well. I believe SIU was ranked at the time (not sure though), and was one of only 3 conference losses for SIU, and 6 overall for arguably SIU's best team ever. At the time, it kept BU in at-large consideration, and ended up being our marquee win of the year.
Individually, it might be the biggest. However time will tell as to whether it has the longstanding effects to make it a truly 'big' win.