I sent the following in as a suggestion but I'm also posting it here for discussion.
I just evaluated Express Talk, Gizmo, 3CX, X-Lite, Ekiga, Mizu, Emansip, Linpoune, Kopanga, and Zoiper. Express Talk is the clear winner with X-Lite a close runner up.
Just one thing...
A local video window is absolutely essential. I need to be able to see my camera output to be sure it is framed correctly and to adjust focus. I also often move the camera around when two or more people are on the call with me. I have no way to do that without local video.
Bill