We gave Ghost Arceus very strong consideration for S-tier when we created this list. We were a bit unsure about it, but agreed it was at the least top A-tier. Hell when we were considering an A+ Tier it was the first thing that came to mind.
Also Sun Blue Fire is literally the only reason to use Reshiram (except for Flame Charge I guess but who the hell uses that). If you're trying to use it anywhere but a Sun team you are far better serviced by Kyurem-W or Rayquaza.