In overall, Gliscor has the highest stat total (510)
It is followed by Scizor (500)
And last but not least is Zangoose(458)
They are all good Pokémon and all three of them have different rolls so you can't point out that one is better then the other in this situation.
Defensively Gliscor is the one you are looking for. It has a monstrous defense stat reaching 383 at Max! In this aspect he seriously outclasses Zangoose that is a frail Pokémon reaching only 240 at max. This isn't much. Scizor has a good defense stat reaching 328 at Max. Gliscor is also the fastest one between the three. It speed reaches 317 at max.
Scizor is in this aspect the black sheep, but it's priority moves make up for this. Coupled with the ability technician, it weak priority moves get a serious power up making Scizor a monster.
If you are looking for an Offensive scout then you are looking for a Scizor.
As mentioned its low speed gets covered due to it's priority moves making Scizor a feared opponent. Zangoose is once again not the recommened Pokémon for this job since it is highly outclassed by Scizor. Gliscor can be a good offensive threat, but it is better as a physical wall.
So I would say Defensively Glicsor is your answer
Offensively, Scizor is the one
Zangoose is a good offensive threat too, but there are many Pokémon that do the job better, like Infernape
Between the three I personally prefer Gliscor, but this is just a matter of opinion
Hope this helped