Five two color decks and the introduction of experience counters, the mechanic that made these commanders scale over a long game.
Experience counters go on you, not on a permanent, and nothing removes them. Each of the five commanders earns them a different way and gets better as they accumulate, so a commander that has died three times still keeps everything it earned.
When a myriad creature attacks, it makes attacking token copies against each other opponent. Designed for multiplayer and worth nothing in a duel.
Evolve puts a +1/+1 counter on a creature whenever a bigger one enters. Ezuri turns those counters into experience, which is how the Simic deck snowballs.
Constellation triggers whenever an enchantment enters under your control. The Orzhov deck uses it as the trigger for its own experience engine.
Commander 2015 is the experience counter set, and that mechanic solved a real problem. Before it, a commander that kept dying got worse. With experience counters, the progress lives on you and survives every removal spell, so committing your commander to the board stops being a risk you can be punished for repeatedly.
Meren of Clan Nel Toth is the deck that outlived the rest. She is still one of the best graveyard commanders in the format and remains a common recommendation for a first sacrifice deck. Mizzix is close behind for spellslingers, since the cost reduction gets absurd within a few turns.
Compared to later batches these decks are slow, but the commanders themselves hold up.
Plunder the Graves is the standout and the best long term investment. Seize Control is the most fun once the discount snowballs and you cast three spells in a turn you had no business casting.
Daxos the Returned
An Orzhov enchantment deck from Commander 2015, led by Daxos the Returned, cast enchantments to generate experience counters and spirit token creatures, then weaponize the enchantment count with Sphere of Safety and Sigil of the Empty Throne.
Meren of Clan Nel Toth
A Golgari sacrifice and recursion deck from Commander 2015, led by Meren of Clan Nel Toth, earn experience counters when creatures die, then use Meren's end-step trigger to return powerful creatures from your graveyard for free.
Mizzix of the Izmagnus
An Izzet spellslinger deck from Commander 2015, led by Mizzix of the Izmagnus, earn experience counters by casting instants and sorceries with higher mana values than your counter count, then reduce all your spell costs by that amount.
Ezuri, Claw of Progress
A Simic evolve and +1/+1 counters deck from Commander 2015, led by Ezuri, Claw of Progress, earn experience counters from small creatures entering play, then spend them to place +1/+1 counters on your biggest threat at the start of combat.
Kalemne, Disciple of Iroas
A Boros big-creature deck from Commander 2015, led by Kalemne, Disciple of Iroas, cast power-five-or-greater spells to earn experience counters, doubling the double strike value on Kalemne for an exponentially more lethal commander threat.