The larger the volume being mashed, the better it will hold temperature, regardless of the material of the mash tun. Geometry also plays a part, by maximizing the volume to surface area ratio, you minimize losses. So for the very best mash tun, you should make massive quantities of wort in a giant sphere.
My experience is that for 5 gallon batches, a kettle just isn’t a good option - unless you leave it in a sauna that is heated to around 150F. In that situation, even a bottling bucket works great as a mash tun.
