Kroxa, Titan of Death's Hunger excels at this, providing a cheap discard effect early that with escape becomes a monstrous, near-unkillable threat later
You always want to have access to all your colors so you can cast any spell that may find itself in your hand
Feral Ghoul I also like Feral Ghoul for its ability to give your opponents a lot of rad counters without giving any to yourself, if you manage to put enough +1/+1 counters on the Ghoul
(Until your next turn, that creature cant attack or block and its activated abilities cant be activated.) Chaos: Whenever you roll Chaos, put X +1/+1 counters on target creature you control, where X is equal to the number of cards in your hand
Creature Artifact - Giant Dirge Bat Mutate (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own
Each of these will reward you with some number of packs from a different Magic: The Gathering set - and, due to how expensive the cards can be even when you're only playing digitally, they're effectively essential to every player both old and new
