I chose Nike since the topic was best running wear, and I have always been a huge fan, of all of their running apparel...it is functional, comfortable, they have such a variety of colors/styles, and their skirts, that I believe are supposed to be for tennis, work out PERFECT for a triathlon. I wore Nike in my 2 1/2 marathons, and for 13.1 miles, they held up really well.
If I can add my comments about a couple of the other brands....
C9/Champion (which I have C9 from Target): these are by far, the MOST comfortable running bra (the long and short version) I have ever worn while exercising. In several of my tri's, I have worn the C9 top with a different brands skirt, because I loved how amazing and comfortable the running bras were. I don't know if this is company wide, or just in Chicago, but in my area, the C9 running bras (I think they're seamless) have been discontinued, and I have no idea why. I even called Champion's headquarters a couple months ago to beg them to not do this.
Skirtsports: while VERY cute with their patterns, and I am sure ok for a running race only, this was not good for a triathlon. It didn't have the bike shorts attached to it like the other brands of skirts that I race in (Nike, C9, Addidas) and so even just that thin layer with the shorts attached was missed...not as much padding, I was sliding all over the place when the bike leg first started since my bottom was wet (had on swim trunks, then skirt), and it blew around like crazy.
I usually only race now in skirts, as they are so cute, and to me, very comfortable, so I am always trying all of the brands and this is what I've found so far...Nike is the best fit.