It's "Rivalry Week" in MLS, which means most teams will end up facing their most-hated foes for a fun week of mayhem. There are a lot of rivalries and regional cups in MLS, but only a few that can be considered the best.
Which are they?

Orlando City vs. New York City FC
This not an official rivalry and there's nothing local about it, but these two teams have crammed a lot of history into their 2+ years in MLS. They both entered MLS in 2015 as expansion sides, which inevitably led to constant comparisons and the desire to prove which expansion team was better.
But then the rivalry got boosted with some personal feelings entering the equation. Jason Kreis, coming off an appearance in the MLS Cup as RSL coach, was tapped to join NYCFC as their first coach and help build their expansion team from scratch. He was unceremoniously fired and replaced with Patrick Vieira after just one season and Kreis later ended up at Orlando City. Now, there seems to be something a little extra when the two coaches meet: Kreis out to prove that NYCFC made a mistake and Vieira keen to show he was an upgrade.
The rivalry took a turn recently, when Vieira claimed NYCFC dominated their May meeting and Kreis took issue with that, accusing Vieira of disrespecting him. Vieira later said he wasn't interested in Kreis' "personal attack."
Given that the Lions don't have any regional rivals in Florida, NYCFC remains their biggest foe. It may be a new and unconventional rivalry, but it is one to keep an eye on.

LA Galaxy vs. San Jose Earthquakes
The so-called Cali Clasico seems to be on its way out as a top rivalry. Once LAFC joins the league next year, the Galaxy will surely be focused on their crosstown rivals. The Earthquakes' best chance at being in another great rivalry will come along if the Sacramento Republic join MLS (which should really happen).
The Cali Clasico has been convoluted a bit by the fact that the original Quakes moved to the Houston Dynamo, and then a new version of the Quakes sprouted up, and then Chivas USA came to LA and became a rival for the Galaxy. The Quakes also just haven't been very good, which hasn't helped.
But, as far as anything remotely recent goes, we'll always remember the way David Beckham bent the ball like, well, himself and smacked Sam Cronin laying on the ground because Beckham thought he was faking. Then, as a scuffle ensued, the Quakes mascot, Q, randomly stumbled into the fray like he got lost on his way to the concession stands.

FC Dallas vs. the Houston Dynamo
This isn't a rivalry that is going to excite neutrals in the same way that the higher-ranked ones on this list will, but you can bet the fans in Texas care about this one. Known as the Texas Derby, this rivalry has stretched back to 2006 and it's been a pretty back-and-forth series over the years.
The best part? The winner of the rivalry doesn't win a trophy – they win an actual, working cannon. The winner gains control of an 18th-century replica howitzer called El Capitan, which can be fired (blanks of course). Whoever wins gets to take the cannon home and fire it off, which is just a fun twist.

Toronto FC vs. Montreal Impact
All the Canadian teams are considered rivals (just like all the Cascadia teams – Vancouver, Seattle and Portland are rivals), but proximity and being in the same conference makes Toronto and Montreal the Canadian derby of choice. Nicknamed the 401 Derby as a reference to the roadway connecting the cities, Ontario Highway 401, the rivalry actually dates back to the 1970s, when clubs teams from the two cities got the hate started. Once the Impact and Toronto FC eventually came into the picture, they picked up where clubs of the past left off.
There's been plenty of back and forth over the years, but last year's Eastern Conference final was a series that will be remembered for a very long time. A whopping 12 goals were scored over both legs and Toronto gained, lost and regained the lead. The matches were wild and the stadiums were packed for one of the all-time classic MLS series.

D.C. United vs. the New York Red Bulls
As far as MLS-only history, this is the oldest rivalry in the league and this was the league's first great rivalry. The rivalry has lost its luster a bit lately because D.C. United haven't been very good lately and Red Bulls fans have started to turn their attention to New York City FC. But the Atlantic Cup, as it's known, has had plenty of great moments over the years.
There was that time Alecko Eskandarian of D.C. United celebrated a goal in 2006 by cracking open a can of Red Bull and spitting it out in disgust, mocking the Red Bulls, who had only recently re-branded and been named after the energy drink company. Or the time Bill Hamid got ejected during a playoff match and later celebrated D.C.'s win by emotionally shouting, "They can't hold us back!" which turned into a popular #TCHUB hashtag. One of the enduring great calls in MLS has to come from Jaime Moreno's 2004 goal vs. the MetroStars: "You just got burned Jonny Walker!"

New York Red Bulls vs. New York City FC
This is one of the newest rivalries on this list and it has a short history, but fans in the Big Apple really turn out for it. Nicknamed the Hudson River Derby, this rivalry got started from the moment New York City was announced as an expansion team in 2015. Red Bulls fans didn't like NYC infringing on their territory, and there's been a debate ever since as to whether New York is red (Red Bulls) or blue (NYCFC).
If you want petty, this rivalry has it for you. Red Bulls fans made one of the rudest tifos we can remember when they basically called Frank Lampard and Andrea Pirlo old and frail. But all any Red Bulls fans needs to say is "7-0" and remind NYCFC fans of the fateful day in 2016 when the Red Bulls crushed NYCFC by a humiliating 7-0 score line. NYCFC haven't had many bragging rights as they've beaten the Red Bulls only once in seven meetings, but that figures to change soon as NYCFC are clearly a lot better than the Red Bulls this season.

Seattle Sounders vs. Portland Timbers
Anyone who says that this isn't the biggest rivalry in MLS is wrong.
It's got everything you want out of a rivalry: Constant one-upmanship and tit-for-tat MLS Cup trophies. Cheeky banter between coaches and players, including a couple of profanities. Dedicated fans and amazing tifos. Spiteful pettiness. And a real genuine dislike between cities that goes well beyond soccer. We already broke down the best moments in the Sounders-Timbers rivalry and there's more than merely makes up that list.
What makes the Sounders-Timbers rivalry so special is that it's been going on for decades – both as a rivalry between cities and between soccer teams. The Sounders and Timbers first met in 1975 when both teams played in the NASL. They would continue to meet over the years in different leagues and as different iterations and the rivalry would get intense once the Sounders joined MLS and the Timbers were still in USL.
It doesn't hurt that the Sounders and the Timbers have two of the best fanbases in all of MLS, but what makes this rivalry real is that they really do wish nothing but the worst for each other on the pitch.