How much damage must I assign to a blocker before trampling over?

You must assign at least lethal damage, which is normally the blocker's remaining toughness, to each blocker before the rest can be assigned to the player.

Does Trample work against a player when the creature is unblocked?

Trample only matters when the creature is blocked; an unblocked creature already deals all its damage to the player, so Trample has no effect.

What is the best Trample card?

By popularity, Rampaging Baloths is among the most-played Trample cards — see the full ranked list above.

Best Trample cards in MTG

Trample: When a creature with Trample attacks, the attacking player may assign excess combat damage beyond lethal to the blockers over to the defending player or planeswalker.

Trample is the answer to chump blockers, ensuring your big creatures still deal damage even when blocked by a small creature. It is especially powerful on large creatures in ramp and stompy decks, and combos beautifully with Deathtouch and pump effects. Always assign just enough damage to kill each blocker, then send the rest through.

FAQ

How much damage must I assign to a blocker before trampling over?: You must assign at least lethal damage, which is normally the blocker's remaining toughness, to each blocker before the rest can be assigned to the player.
Does Trample work against a player when the creature is unblocked?: Trample only matters when the creature is blocked; an unblocked creature already deals all its damage to the player, so Trample has no effect.
What is the best Trample card?: By popularity, Rampaging Baloths is among the most-played Trample cards — see the full ranked list above.