Optima, Bold, lowercase m.
It's always easy for me to pick out Optima because although it's a sans-serif, it has a bit of shape to its form. You can sort of see how each leg of the 'm' is not a perfect rectangle, rather they have a slight curve to them from the top of the leg to the bottom (much like flare jeans.) Wikipedia calls it a "swelling at the terminal" which is really a great way to describe it, swelling. The swelling in Optima helps differentiate it from every other sans-serif. Does this uniqueness make for better readability? I don't know, but it does help increase its celebrity in the type world. You can't be famous is no one knows your name. (Note after reading Wiki article, this calligraphic touch to Optima does in fact help legibility!) And speaking of names, Optima also goes by Zapf Humanist because it was a humanist sans-serif created by Hermann Zapf. (You may know Zapf from Zapf Dingbats, he's a big player in the type game.)
I also enjoy Optima 'm's humps. While, I don't mind the slight curve/dip at the very top, starting point of the letter. Although, it becomes much more pronounced at the terminals, particularly in the Black weight (at which I find myself disliking the shape more and more.)
Courtesy of Wikipedia.com:
Design Foundry: D Stempel AG
Designer: Hermann Zapf
Classification: Humanist Sans-Serif
Optima is the typeface used on the Vietnam Veterans Memorial.
Humanist typefaces include Calibri, Johnston, Lucida Grande, Segoe UI, Gill Sans, Myriad, Frutiger, Trebuchet MS, Tahoma, Verdana and Optima, a.k.a. Zapf Humanist. These are the most calligraphic of the sans-serif typefaces, with some variation in line width and more legibility than other sans-serif fonts.