You have several nice choices here. I like the balance and the font choice. We are looking for a pictoral or abstract mark, so at this point #21 is my favorite. I would like to see some more ideas along those lines.
The image needs to be more symmetrical and the "8" not quite so wide. Also, I think that the image would benefit if the "T" was as wide as the 8. And, maybe use an "I" with the top and bottom cross pieces in place.