Really hard question, oooof. Generally I like unwilling TFs the most, but particularly when the unexpected changes are eventually enjoyed by the transformee (not a big fan of bad endings although they're not a turn-off if the TF and the scenario are hot). The main kind of willing TFs that I think I enjoy just as much are those in which the transformee voluntarily undergoes an humiliating, extreme and/or degrading transformation for someone else; usually this ties in with a submissive/dominant relationship dynamic.