Most of the bikes are 12mm, I think with very few exception on the four strokes.
Full description is 12mm metric fine, I believe, as it's 1.25mm thread pitch. Straight metric, again I believe, to be 1.5mm pitch which is normal engineering size for non specific use.
The need to be a little careful is there if you wish to avoid a mash-up.
I've no recommendation as my screw in type is an old Sykes-Pickavent which I can't find a current example of.
The one's with rigid tube and rubber cone that you just hold in the plug hole while you turn it over are a little awkward but they do work well and have always given me repeatable readings.
You can assess if they are giving realistic readings by trying them in another known good engine to see what they read.
