Gengar should be an S because of its variety. There are countless numbers of gengar sets which support, wall and attack. Until you know what set a gengar is running, it can be unpredictable, and tricky to play around. The lack of predictability because of its huge movepool imo makes it an S Rank.
Slowbro is probably the best physical wall we have right now which is a safe switch, has instant recovery (although not status recovery, and offensive pressure. Unless Slowbro checked or countered immediately it has free reign to regenerate health, and continue shelling out stab moves. Its defensive safe switches imo give it a place in S rank. I could also see it as a suitable A+ because of its great defensive characteristics, but need for a cleric.
I see Gyarados as an A+ because of the support it needs. It is very powerful, even forcing some pokes to run an unconventional hp electric. However, I don't think its offensive ability is S worthy due to the need to set up, despite bulk/ability.