I like Signature, YLI and King Tut all in 100% cotton. It just depends on what you and your machine like. I use a lot of variegated threads too, they blend well on any of the colors of the blocks. I usually use the same in top and bottom, but others have use different types in top and bottom and different weights of thread too.
Also, you stated that you had your fabric as tight as you could get it on the poles. You don't want your sandwich to be real tight, you do need some give, back it off at least one notch if not two on the pole. That may be why your needles are breaking. Also if you start having problems with thread fraying, try a topstitch needle. The groove and eye are bigger and put less drag on your thread while it goes through the sandwich. I have gotten so I only use topstitch needles in all of my sewing/quilting/embroidery because of this.