These cards just increases the mana cost of your opponent's spells, and while Thalia, Guardian of Thraben is a symmetrical effect (it affects you too), it is an extremely good card to drop in Turn 1 or 2 while your opponents are trying to play mana rocks or ramp their lands
Most of the rarest MTG cards come from Magic's earliest sets (Alpha and Beta) and appear on the Reserved List, which Wizards of the Coast has promised never to reprint
Aang, at the Crossroads Destined Savior
Black As someone building an Edgar Markov deck (I know, how original), Stensian Sanguinist looks ideal
Having this on board means that your crimes interact with their plan while furthering yours this our aforementioned ideal scenario
Court-style cards generally allow their owners to become the Monarch, and offer an additional payoff that occurs on each of that players upkeeps