... Ok im sorry about it but, hello? Kangashkan > Gengar? Scizor the 3rd best mega?
I wont deny the Kangashkan might be an easy top 5 megas due to the sheer brutality Power Up Punch brings, along with the ablity to bypass subs and top notch bulk. However, there is a reason why Gengarite was already speculated to being banned with the sole view of its base stats and ability, trapping itself is quite the horrible experience for the victim since more often than not youll being doing nothing against the trapper while they take a pokemon w/o you being able to do anything, but yeah the most common (offensive) trappers we had until now were Dugtrio and Magnezone, which CANNOT apply their trappiing to all pokes and are mediocre at handling most others. Gengar not only can do just this (trapping 90% of OU) but it hits as hard as a non-Specs/LO Kyurem W, arguably on the best typings right now with the introduction of Fairy, decent enough bulk and super as ever movepool.
Now on the Scizor statement, i like to overhype the Megas because all of them have potential and bring variety to the table (yes, even you Gyara), however, Scizor, as much as i like his potential as both an offensive and defensive SD late game sweeper, is not nearly in the leagues of monsters such as Gengar, Blaziken or, one you mentioned, Kangashkan. That is because his main role, CB revenge killer and pivot, is completely outclassed by regular sciz, also, people have come up with mind games including Assault Vest Sciz, which canactually do its job reliable at trapping lati@s and non-HP fire gengar, which M sciz cannot do.