5D Mark II, because it introduced full HD video (1080p) to digital SLR cameras. This feature was just a last-minute addition to the camera's already impressive photography capabilities. Had Canon known that it would have proven to be such a popular and demanded feature, they might have introduced the C300 camera much earlier.
The Canon Rebel series of cameras priced well under the $1,000 mark was the next to be videofied. Now, many professionals all over the world have switched to Canon DSLR cameras. Why are so many abondoning cameras that cost thousands more to shoot video on cameras that were never even meant for taking video? How did Canon's accidental pro video camera become so popular?
Digital SLRs with video was probably the biggest change that independent filmmakers have seen since mini-DV cameras came out over a decade ago. Three things about Canon DSLRs changed everything: their large sensor size, low price and their ergonomics.
Now filmmakers could get a camera that could give them the forever coveted narrow depth of field (i.e. when background is out of focus while the foreground is in focus for artistic emphasis). This all in a camera that costs $10,000 or so less than an equivalent camera (at the time it came out), and in body small and light enough to fit on a Glidecam 2000 Pro. Now the impossible was possible by accident.
One other feature that I didn't mention is that they introduced a cheap file-based workflow to video. Although it's clunky compared to a Panasonic P2 camera or XDCAM from Sony, it's certainly better than the process of shooting on a RED camera as far as time goes. Shoot it on an SD card, then either take it to an intermediate codec, such as ProRes or the new freely available Avid DNx codec, or take it straight into a compatible editor that can handle the camera's H.264 files (e.g. Premiere CS5.5 and up).
If you haven't looked into the Canon Rebel series (starting from T2i and up), you should look into it. While I don't expect these cameras to be right for everyone (they certainly are not for everyone), they will change the ways of other competing manufacturers. I expect that the bar will raised and the price lowered for future cameras, because of DSLRs.