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 Evolution Witness

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 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.

Infinite +1/+1 counters on a creature

Basking Broodscale

Arwen, Weaver of Hope

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}.
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

Combos with "Reyhan, Last of the Abzan"