Cast the creature with MV<=2 by paying its mana cost if it doesn't have {X} in its mana cost, or by paying enough mana where X=1 otherwise, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Ashnod's Altar by sacrificing the creature, adding {C}{C}.
When the creature dies, Reyhan triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.
If the creature can be cast for {C} or less, this combo nets infinite colorless mana, and you may cast that creature for an arbitrarily large amount of mana if it has {X} in its mana cost.

Infinite +1/+1 counters on a creature

Infinite colorless mana

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Infinite storm count

Creature with MV<=2 that is colorless and enters the battlefield with a +1/+1 counter on it

Reyhan, Last of the Abzan

Evolution Witness

Ashnod's Altar

Cast the creature with MV<=1 by paying its mana cost if it doesn't have {X} in its mana cost, or by paying {1} otherwise, causing the creature to enter the battlefield with a +1/+1 counter on it.
Activate Phyrexian Altar by sacrificing the creature, adding {1}.
When the creature dies, Reyhan triggers, putting a +1/+1 counter on Evolution Witness.
Evolution Witness triggers, returning the creature from your graveyard to your hand.
Repeat.

Creature with MV<=1 that enters the battlefield with a +1/+1 counter on it

Phyrexian Altar

Cast Basking Broodscale by paying {1}{G}, causing it to enter with a +1/+1 counter due to Arwen.
When Basking Broodscale enters, it triggers, creating an 0/1 Eldrazi Spawn creature token that enters with a +1/+1 counter due to Arwen.
Activate the Eldrazi Spawn by sacrificing it, adding {C}.
When the Eldrazi Spawn dies, Reyhan triggers, putting at least one +1/+1 counter on Basking Broodscale.
Basking Broodscale triggers, creating an 0/1 Eldrazi Spawn creature token that enters with a +1/+1 counter due to Arwen.
Repeat from step 3.

Basking Broodscale

Arwen, Weaver of Hope

Cast Basking Broodscale by paying {1}{G}.
When Basking Broodscale enters, Good-Fortune Unicorn triggers, putting a +1/+1 counter on Basking Broodscale.
Basking Broodscale triggers, creating an 0/1 Eldrazi Spawn creature token.
When the Eldrazi Spawn enters, Good-Fortune Unicorn triggers, putting a +1/+1 counter on the Eldrazi Spawn.
Activate the Eldrazi Spawn by sacrificing it, adding {C}.
When the Eldrazi Spawn dies, Reyhan triggers, putting at least one +1/+1 counter on Basking Broodscale.
Repeat from step 3.

Good-Fortune Unicorn

Cast Scurry Oak by paying its mana cost.
When Scurry Oak enters, Good-Fortune Unicorn triggers, putting +1/+1 counter(s) on Scurry Oak
Scurry Oak triggers, creating a creature token.
When the creature token enters, Good-Fortune Unicorn triggers, putting +1/+1 counter(s) on it.
Activate Ashnod's Altar by sacrificing the creature token.
When the creature token dies, Reyhan triggers, putting +1/+1 counter(s) on Scurry Oak
Repeat from step 3

Scurry Oak

Cast Scurry Oak by paying its mana cost, causing it to enter with a +1/+1 counter due to Arwen.
When Scurry Oak enters, it triggers, creating a creature token that enters with a +1/+1 counter due to Arwen.
Activate Carrion Feeder by sacrificing the creature token.
When the creature token dies, Reyhan triggers, putting a number of +1/+1 counters on Scurry Oak
Scurry Oak triggers, creating a creature token that enters with a +1/+1 counter due to Arwen.
Resolve the Carrion Feeder ability.
Repeat from step 3.

Carrion Feeder

Activate Bartolomé del Presidio by sacrificing Reyhan.
When Reyhan dies, its persist ability and innate ability trigger.
Resolve the persist trigger, returning Reyhan from your graveyard to the battlefield without a -1/-1 counter on it due to its replacement effect.
Resolve the innate trigger, putting a number of +1/+1 counters on a creature.
Resolve the Bartolomé del Presidio ability.
Repeat.

Combos with "Reyhan, Last of the Abzan"