3 and a Black, Green, and Blue.

Legendary Creature - Elemental Avatar, 6/6

No combat abilities... why should we care about this commander?

During each of your turns, you may play up to one permanent card of each permanent type from your graveyard.

Oh my...

Your Graveyard and You

For Muldrotha, the graveyard is a second hand of cards. Self-milling from our own library is almost equivalent to drawing those cards. Discarding has little detrimental effect on our card options. The larger our graveyard, the better our toolbox.

Intentionally leaving a big chunk of our deck in our own graveyard does expose our strategy to hostile graveyard removal or exploitation. Our green cards can help remove the enchantments that exile like Rest in Peace and our blue cards can help with countering spells or the occasional effect. That still leaves the yard wide open for The Scarab God to pick through and steal our buried secrets. However, if we want to learn some tricks on how to fill our own graveyard (along with everyone else's), then Scary might have some ideas on that.

Permanents

In general, Muldrotha should not be using an instant or sorcery if a permanent exists that can do the same thing. Permanents can be recast from the graveyard, while instants and sorceries cannot. If you insist on including Villainous Wealth because it is a Sultai deck and you believe every Sultai deck must include Villainous Wealth (which is true), then consider something like Eternal Witness or Archaeomancer to be able to do it again.

Creatures

It is easy to view Muldrotha as a deluxe blue/black/green version of Karador, Ghost Chieftain or Meren of Clan Nel Toth. When two of the most popular EDHREC commanders get a new version with a stronger ability in arguably better colors, that's an indicator that the popularity chart might be getting a bit of a shakeup soon. If you have already memorized your own favorite multi-branching tutor flowchart for your Birthing Pod deck, then Muldrotha might help add another harmonizing voice to the fugue.

Do you want to cast Spore Frog every turn? Muldrotha can do that. Fleshbag Marauder? Good to go. Sakura-Tribe Elder? Yes, but we've already done this stuff before and it's getting boring. OK, well now that we add blue how about Siren Stormtamer or Glen Elendra Archmage? Did you notice that each creature mentioned above has the word "sacrifice" on it? That's something to keep in mind when searching for Muldrotha cards.

Depending on the power of the rest of the creatures we include, it could be cute to use both Meren and Ezuri, Claw of Progress in the same deck, since they can play off of the same bank of experience counters.

Artifacts

Silas Renn, Seeker Adept and Glissa, the Traitor both have strong opinions about artifacts and their movements in and out of their graveyards. Hey! What if they could be played in the same deck? Throw all our favorite "Capsule" and "Spellbomb" cards in there and cast them and blow them up again and again. Ratchet Bomb? Engineered Explosives? Boompile? Congratulations! You are now on a government watchlist for having read this paragraph.

Remember that Muldrotha can cast an artifact creature as either the artifact or the creature for the turn, giving a bit more flexibility among the permanents of each type. Do you have an Arcum Dagsson deck that wants to add black and green?

As an aside, the same permanent type principle applies to enchantment artifacts, enchantment creatures, and to an indirect extent artifact lands if you are already including other support for artifact recursion to hand.

Lands

Titania, Protector of Argoth and The Gitrog Monster are well-versed in the tectonic movements of lands through the underworld. Undead lands? The point is, if you already have a nice combo involving Crucible of Worlds or Ramunap Excavator, then Muldrotha can probably help. Cards with the dredge ability like Dakmor Salvage and Life from the Loam give their own recursion while also filling our own graveyard with more cards for Muldrotha to cast.

If you have some type of fetch land in your graveyard, you can play it once every turn and get a land out of your deck. Practice your deck shuffling so you can do it quickly because shuffling will be frequent and your opponents may target you just for taking too long.

Alternately, you could activate a Strip Mine, Wasteland, or Ghost Quarter to rough up someone else's mana base and then play that again out of the graveyard every turn.
 

Enchantments

Graveyard recursion for enchantments tends to fall outside of Muldrotha's colors and firmly into white. However, many enchantments have the ability to sacrifice themselves to our own benefit. Whether they accumulate counters like the "Quest" or "Ordeal" cycles, or they just wait to be sacrificed like the "Vessel" and "Seal" cycles, we can use them again and again.

Aura enchantments tend to see the graveyard more often than regular enchantments due to their natural dependence on removable creatures, so it may be easier than usual to reuse them. Totem armor?

Anything with a cumulative upkeep can be buried and recast whenever the upkeep costs more than the spell itself. Elephant Grass or Mystic Remora for a single mana per turn for the rest of the game. Alternate the recursion on odd and even turns, and we can maintain both of them for just two mana per turn, forever.

Planeswalkers

For this section, we should be looking for low-CMC planeswalkers that have a decent minus ability, so that we can burn out their loyalty and recast them cheaply. While we could include every version of Jace and Vraska if we wanted our own slash fan fiction romance theme deck, where Jace makes two copies of himself and they kneel in unison, gazing with a fierce yearning in Vraska's direction as she seductively demands their obedience. Not that anyone here is thinking of doing that...

Anyway, the higher the mana cost of any one permanent, the less mana we have left to cast our other permanent types and miss out on Muldrotha's recursion value.

Kiora, the Crashing Wave, for example, can be activated with the -1 ability to draw a card and play another land. Then on our next turn she can be activated, buried for having no loyalty left, recast from the graveyard, and activated again to draw two cards and play two more lands that turn.

The Deck

While it would be easy to take every legendary creature mentioned in this article and throw in a few of the top cards from each commander's page, I'm going to do something a bit different. Since my deck plans to mill my own library, why not mill every library and make that a win condition?

This deck is somewhat tuned toward the competitive side of the spectrum, as indicated by the multiple 1-CMC green creatures that tap for mana and the expensive fetch lands. If you want a more casual (slower) build or a more budget build, you can adjust those categories of cards respectively. Alternately, you could swap in some 0-CMC mana-producing artifacts to make it faster or add tutors to make it more consistent. It still won't generally combo out to an instant win, unless you count an explosive mana ramp into one or more of the three X sorceries as a combo. This deck plays similarly to my Damia, Sage of Stone deck, featured on Star City Games' Commander Versus series, with less landfall and with most of the instants and sorceries removed or converted to equivalent permanents... but I'm still playing Villainous Wealth.

If you want to hear more of me rambling on about Muldrotha, you can watch episode 20 of the Commander Time podcast and see a picture of cards piling up as we talk about them. For more on a comparison between Meren and Karador, you may like this article by Joseph Schultz.

Enjoy...