This passive ability alone would make Tinybones worth playing, but the fact he can force each opponent to discard for four mana is the icing on the skeletal cake
Nissa, Vastwood Seer is my personal fave, a Civic Wayfinder that transforms into a sweet card advantage engine with admittedly lackluster minus abilities
Its super effective as a Voltron commander because it gets to attack four times a turn cycle, so you basically get three additional combats a turn
Olivia Voldaren is a card that dominates games and is able to end them in a wide number of ways
When a Spell/Trap Card, or monster effect, is activated (Quick Effect): You can Tribute this card
On the other hand, thanks to their scarcity, Secret Lair cards can end up costing far more than regular copies, especially if they have super sweet art