Greater. A guess
I say Greater. Notice the flat head. Lesser tend to have a more pointed head. Also, very white on the flanks. Lesser tend to be more greyish.
This is a tough one. I wouldn't call it based on these images. I'm leaning toward Greater based on the first image. The forehead slopes gradually into the bill, like a Canvasback, not a Redhead (Greater vs. Lesser Scaup). The eye placement is a bit more like Greater, as is the head shape. The bill is rather long for a Lesser.- Bob Lewis
Pretty clear Greater. More humped head vs peaked, lighter gray back, small dark nail on tip of bill, and then the last shot is great, showing the classic thick cheeks and noggin of the Greater. I think that last photo is good enough for ID by itself.Andrew Thornton
Lesser. Greaters have "black lipstick" that extends beyond the nail on the bill across most of the front part of the bill. It doesn't look like your bird has that.
Def. Lesser. Head does not show any green, but clearly shows a blue hue, and it is clearly illuminated. Gray visible in white flanks (not brightly white). And peak visible in both of first two photos. Fun.
I would lean toward Greater based on shape of the head and bill. Photos aren't good enough to id on other points.