Yeah, I'll list up the reasons later."BIGGER THREAT TO MY TEAM"
Now I understand. Just because your team completely demolishes a Pokemon does not mean it isn't viable. As most people know, Zekrom is my favourite Pokemon within the tier, and if the tier analysis was written to my team as you seem to be doing Lugia and Pogre, to name a few of the viable mons Zekrom crushes, would be at D rank at best and Zekrom would be at S+++. However, the tier is not written to my team, for good reason, just like it isn't written to your team. It is written to the majority, and if you want a mon promoted/demoted you should address your pitch to the majority of teams while assessing whether or not your promotion/demotion is necessary.
I still say that Ditto deserves to be B, as even though it requires a lot of team support, it greatly limits your opponents options, and can sweep mid to late game. It can be sent as a lead to scout for your opponents movesets, it is one of the best revenge killers in AG, and it can copy boosts like Swords Dance on Arceus, or Geomancy on Xerneas. Just by the presence of Ditto, your opponents will likely feel discouraged from using set up moves, which could win you games. I can't think of too many instances where Ditto is useless, in a hyper offensive metagame like this. (AG is hyper offensive, right? Or am I doing it wrong?)