1D Mark IV - first APS-H DSLR with HD video
Digital Rebel / 300D - First DSLR affordable for the masses
1Ds - first full-frame DSLR
5D Mark II - first "affordable" full-frame
EDIT: If I win, I would prefer the "UNDERSTANDING THE CANON 1D MARK IV" video, second choice "UNDERSTANDING THE CANON SPEEDLITE 580EX II / 430EX II".