For four mana, Sorin the Mirthless starts with four loyalty and the option to use either his +1 ability, allowing you to gain card advantage in exchange for life, or -2 to create a 2/3 black Vampire with flying and lifelink
Since I'm not as familiar with some of these decks, I don't want to make assumptions about their viability, so I'll list them down here
Also, changelings don't count
Chukov Standard Not Legal Alchemy Not Legal Pioneer Not Legal Historic Not Legal Modern Not Legal Brawl Not Legal Legacy Legal Comp
Top 10 Most Valuable Cards Gengar Master Ball at 74,800 leads the pack a Rare-base card outpricing every SAR in the set through scarcity alone
Still, this last part should be up to the discretion of the event organizer, but chances are that this isnt something you should expect to see in a Pro Tour or any competitive setting