This post is mainly meant to spark discussion and less so to provide a closing argument on why certain pokemon should be in certain tiers.
What I'd like to add to the discussion is that stall seems to be a very underrated playstyle according to the current viability ranking. Of the top 40 pokemon (S through A), only 10 are used on stall teams, 5 of which can run offensive sets too. The fact that pure stall is one of the most dominant strategies on the higher ladder makes me wonder why some of the amazing stall monsters aren't higher up on the tier list.
One defensive pokemon I'd like to see rise from A to A+ is Slowbro.
Slowbro perhaps fails to perform at S-rank level as it has a few weaknesses to relatively common types such as ghost, grass and electric.
However, this trait is largely nullified by the fact that it has one of the best abilities a defensive pokemon can have which minimizes the costs of switching in and out. Next to that there are a number of top-tier pokemon that have great synergy with slowbro such as heatran, venusaur-mega and gliscor who can all take hits that slowbro would be afraid of having to take.
Slowbro is one of few pokemon that can easily switch in to a lot of the metagame's physical attackers and scare them out. Even after a failed prediction slowbro doesn't have to take a huge chunk of damage as even an adamant pursuit from a bisharp only does ~50%, 2/3rds of which get healed right back up by regenerator.
Slowbro, being such an unbreakable wall if played well against even hyper offensive teams, should definitely be considered for a top-tier rank.
The second pokemon I'd like to see rise is Chansey from B+ to A.
Chansey is almost a staple on a lot of stall teams, being able to check if not counter most pokemon who are purely specially based in the tier. The only thing chansey needs in terms of team support is a physical wall, preferably a fighting resist.
The reason chansey should be in A rank is that it fits a role that no other pokemon in the tier can fill: check nearly half of the tier. With a monstrous HP stat it can even take some physical hits from weaker pokemon. It has reliable recovery and great support options, being one of few viable clerics in the OU tier and also having access to stealth rocks.
Honestly the only thing that you could argue against chansey is that it's not doing too much when on the field if rocks are already up and can be a set-up fodder. However, even at +4 most specially based pokemon can't even 2HKO the blob with full investment while the blob can continuously chip away at them with seismic toss. With this pokemon being a helpful addition to nearly every stall team it's hard to see this pokemon not being somewhere in A rank.
