I suppose off-screen would automatically be the worst but I'm talking techniques and to me, its a tie between a dissolve transformation and a warp transformation I think just dissolving is worse because I actually have seen the warp effect done well dissolving just seems lazy to me
My very very less favorite transformation is when the "creature" burst out of the previous from. Like in Vanhelsing, the werewolf literally rips off the human's body/skin to "get out of the shell". I don't like that!
Personally, type wise, my least favorite are goo/latex type of transformation.
Most of the time, it's only used to hide the transformation and the details.
A dissolve transformation can be useful if it is used in the right context. Finding the right context for this transformation can be challenging. One way to make the transformation useful is by dissolving a person upon capture and reforming the person once their location has changed.