The ‘new’ kid on the block since Grand Prix Seattle is Eldrazi Stompy:
Versus Eldrazi Stompy (and for posterity’s sake, here’s the Eldrazi Stompy list that I like the best).
Eldrazi Stomp — Legacy | _Barook_
Creatures (23)
3 Matter Reshaper
4 Eldrazi Displacer
4 Eldrazi Mimic
4 Endless One
4 Reality Smasher
4 Thought-Knot Seer
Spells (13)
2 Dismember
2 Warping Wail
3 Lotus Petal
4 Chalice of the Void
2 Umezawa's Jitte
Lands (24)
2 Brushland
2 City of Traitors
2 Karakas
3 Eye of Ugin
3 Mishra's Factory
4 Ancient Tomb
4 Cavern of Souls
4 Eldrazi Temple
Sideboard (15)
3 All Is Dust
1 Dismember
4 Leyline of the Void
2 Spatial Contortion
2 Thorn of Amethyst
1 Warping Wail
2 World Breaker

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I did not come up with this list. I found it on the Source (a well known Legacy forum) being worked on by someone who goes by _Barook_ on that forum. It seemed to have very well thought out plans, and also seemed to be more balanced; i.e. it wasn’t all-in on beating Miracles Game 1 (a lot of the colorless versions of this deck focused on 4 Chalice of the Void plus 4 Thorn of Amethyst in the main which meant they were much weaker versus creature decks).
However, I view it as a very favorable matchup for the following reasons:
Maze of Ith is excellent versus giant monsters.
They don’t have much disruption for a fast 20/20 without Karakas or Eldrazi Displacer.
Glacial Chasm can ‘Fog’ them for a few turns.
With Lands versus the above Eldrazi list, here’s how I would sideboard:
-1 Manabond
-3 Crop Rotation.
+4 Krosan Grip
I just want access to a way to answer Chalice of the Void (or Leyline of the Void). They generally try to play Chalice of the Void on 1 (stop Gamble / Crop Rotation) versus you in order to buy enough time to Chalice of the Void on 2 (to shut out Loam). Cutting some number of 1-drops to get around this is a good hedge. It’s also more difficult to try to make a fast 20/20 in the face of Eldrazi Displacer and Karakas.
For those of you interested in the above Eldrazi list, here’s a somewhat in depth sideboard guide I found here.
Versus Jeskai Miracles:
There are two ways to approach this matchup.
a) You can either try to shove a 20/20 down their throat, and hope they don’t have a White removal spell (Terminus or Swords to Plowshares)
b) You can try to assemble many Rishadan Ports (plus using Thespian’s Stage to copy Rishadan Port) to shut off all of their White mana to make sure the coast is clear for the 20/20.
Plan a) is hard to pull off, and also requires you to have access to Life from the Loam to recover on the (very likely scenario) that they do have a Terminus or Swords to Plowshares for your 20/20.
Versus Delver variants:
The plan is straightforward, try to deny them mana, and then search for a Tabernacle to Wrath of God them. They normally have no good response to a fast 20/20 either.
It normally doesn’t improve much for them post-sideboard either, but be wary of cards like Pithing Needle and Surgical Extraction.
Versus Shardless Sultai:
The plan is relatively straightforward. Their deck is much slower than a Delver variant, so the mana denial is even better here. Just be wary of not making a 20/20 into Jace, the Mind Sculptor or Liliana of the Veil. They can sometimes chump block with a Baleful Strix, so ideally you’d have a Punishing Fire to pick that off as well.
Versus Infect
For posterity’s sake, here’s the list of Infect that appeals to me. Gainsay is Andrew Cuneo’s MTGO handle, so I have the utmost respect for this list.
I generally approach this similarly to Delver matchups. Trying to ‘turtle’ up behind a bunch of Mazes of Ith is a bad idea because one Vines of Vastwood cast on their main phase defeats multiple Mazes of Ith. I like to aggressively attack their mana and search up the Tabernacle to Wrath of God them.
Versus Death and Taxes:
You are the control deck here. Given enough time, you can exhaust their multiple Karakas with Wasteland recursion and make enough 20/20s to end the game. You need to manage your life total with Glacial Chasm and Maze of Ith. Post-sideboard can be a lot trickier, since they can shut off your recursion with Rest in Peace. Pithing Needle can also be a nuisance (naming Thespian’s Stage).