Which single (first) ND Filter question

After blowing out some pics of my daughter in this blazing FL sun even at iso 100 and 1/8000, I'm thinking of getting an ND filter for portraits. I mainly use the 2.8II but I also like my 1.4 Sigma 85. Just to try out ND filters, which would you recommend first? 2, 3, 6 stop? I don't think I'll be using it for landscape so I know I don't need the 10 stop ones and I have a B&W CPL as well and hope to use it in conjunction with the ND for additional effect. I want to be able to shoot at 2.8 or 1.4 depending on lense and stay close to wide open as possible while not blowing out the sky.

Youre going to run into some problems here. In order to take pictures of your daughter while having her and the sky properly exposed, youre going to need some high power flash. Anyway, look into a singh-ray adjustable ND filter. They are expensive but Ive never heard anything bad about them.

I have a 3 stop filter and when I shoot video, I need to get the shutter down to 1/50. If I want to shoot at f/1.4, I'd need a 6-7 stop filter. Shane is correct, get a 77mm Vari-ND and get a 72-77mm step up ring for your 35mm.

Why not move the shooting location or time of day to when there is much less blazing sun? No one looks their best at high noon out of doors.

Was the whole image blown out, as the scene was front lit? or was it the background that was blown out because she was heavily backlit?

In the first scenario, a 2 or 3 stop might help you out. In the second, no ND filter is going to help, because its effect will cover the entire frame and the large dynamic range difference will still be there. In that situation you need a good flash/strobe, as another poster mentioned. Then you expose (approx) for the background and use the flash/strobe to illuminate your subject.