Superior Threads Invisifil polyester invisible thread. I used it in a wall hanging with a 60 weight bobbin thread, and it was great. Required a very loose tension though. On my HQ I usually go through all three tension holes with 90% of threads. Invisifil only needed one or two holes (I have forgotten which) plus a significant lightening of the main tension know. I used a 14 needle, tough it would have taken even a smaller one.
Pam