I dragged the photo into my iPhoto to enlarge it and sharpened the image a bit.
The second strand from the top looks the best to me. It has a few larger pearls in front center (unlike the bottom strand which is more uniform in size), the shapes seem a bit more homogenous than the other strands and it has nice luster.
You are going to town with pearl purchases! All good choices...I like the second from the top as well. The third, which seems to have unusual shapes (not as uniform) is probably not a good choice for you.
I asked for another shot of #2 and #4. After he and husband weighed in, I decided on #4 (the bottom one here). Thanks for weighing in! I'll post a picture when I receive the strand (18 inch).
Here they are. They're very nice, uniform, ripply and shiny and just right for casual dress. I'm sure I'll get a lot of wear out of them. Maybe tomorrow I'll take a neck shot. They look a little grey in the photo but in real life they're shiny white.