I placed the blue dragon second on my personal list because I think it is an incredibly strong threat in the current metagame that is able to dodge most common removal spells such as Lightning Bolt and Prismatic Ending
Similarly, permanents that phase out with counters phase in with those counters
I think this will be cracked in one exact deck I cant imagine just now and trash everywhere else
As long as your deck runs heavy on draw and discard, you fill your graveyard up with expensive spells (or cheap ones simply by casting them), then once this big boy hits the battlefield, you can start reigning terror on your opponents by casting those big spells from beyond the grave for free
Also look out for cards that generate a stream of expendable tokens like Bitterblossom, and creatures whose value is not in staying on the battlefield but in entering it, like Bone Shredder
That makes them a considerable mana cost to play, but are worth it in the long turn