Blightbeetle + Phyrexian Altar + Coveted Falcon

Blightbeetle

Phyrexian Altar

Coveted Falcon

How it works

1. Activate Coveted Falcon by paying 1, turning it face up.
2. Coveted Falcon triggers, causing an opponent to gain control of Blightbeetle and causing you to draw a card.
3. Activate Phyrexian Altar by sacrificing the undying creature.
4. When the undying creature dies, its undying ability triggers, returning itself from your graveyard to the battlefield without a +1/+1 counter on it due to Blightbeetle.
5. Repeat from step 3.

This clever loop turns Coveted Falcon into a permanent-swapping engine while relying on a hidden (though unlisted in the raw steps) undying creature to act as your recurring fuel. By turning Coveted Falcon face up, you hand your Blightbeetle over to an opponent, which is where the magic happens: Blightbeetle's static ability prevents creatures your opponents control from receiving +1/+1 counters. When you feed your undying creature to Phyrexian Altar for mana, Blightbeetle's passive protection ensures the creature returns to the battlefield counter-free, allowing you to sacrifice it indefinitely for infinite mana and triggers.