Deoxys-Attack D -> A- or B+
Not sure why this is down in D lol.
You answered it yourself. Also not to mention, there's Imposter-proof lategame cleaners with fewer weaknesses/support issues, such as Quick Feet Mega-Scept / Mega-Ray / Skymin (very niche compared to the other two, but still has a spot), all three of which can function as revenge killers and fast checks to even some stuff that's boosted their speed. I think even Speed Boost + Tail Glow Kyu-W requires less support to get in safely and start cleaning.however, it isnt without its flaws. it needs a lot of team support: defog/spin, slow pivot moves to get it in safely, and a powerful priority user for if it gets impostered after its sash has been broken. also, its not as strong as other protean mons due to the lack of a boosting item, so it can sometimes miss important kills.
I can see maybe C, but not A or B. A-ranks should require little support than most Pokemon and B-ranks shouldn't need much beyond removing Imposter, Sheddy, and specific checks or counters. Dialga's a good example of a B-rank because its bulky, hits hard, and pretty much only needs Sheddy, Imposter, Groudon, and Xtwo removed for it to do its job effectively.