There are no good HM slaves in this gen but you don't need any. Just grab a monkey, chipmunk, or kitten and teach it cut and you're done. You should use the aforementioned for Fly, and You should have surf on your team.
Goruugu and panpour both make very good HM slaves, gives you all 6, water ones + cut on the monkey, fly/strength on the ghost.
and if you evolve panpour, it actually pulls its weight fairly well with surf.
while fly is actually a good move on goruugu since they dont switch, and gives you ground/flying (he can also learn flash, which seems more useful then dive is as a HM anyway, and is a useful move for taking down hard mons by spaming potions, and using when they miss to drop them lower or attack).
before you get the ghost robot, just use what ever flyer you can find.
the dog also makes good use of strength before then too.
but he is right, strength just is for shotcuts and a few items. HM slaves arnt needed if you dont want them.