So, I've been looking at a lot of mons in the current metagame, and I'm gonna go ahead and name a few that I think are really strong.
Hitmonlee/Hitmonchan
Both the Hitmon-Kicker and the Hitmon-Boxer are really good in this meta with the buff of Black Belt, exclusive to the two beforementioned mons and Hitmontop, giving them a marginal boost (I think 50%? Maybe 30%, idk) in Attack and Speed each. It's a great tool for Hitmonlee especially, who was given Swords Dance in DM, acting as a fantastic mid- to late-game cleaner. On the flipside, Hitmonchan is an impressively bulky revenge killer with access to Iron Fist, boosting Drain Punch, Ice Punch, and Mach Punch's power to more than acceptable levels. They also pack a Stealth Rock resistance, letting them swtich in often. Good counters I have found tend to be bulky Specs Starmie, Bulky Gothitelle (preferrably Specs. She's a monster, too.) and Physically Defensive Suicune.
Raikou
Raikou was given Weather Ball in DM, making it very scary in the Rain, providing a 100BP Water-Type move that dispatches its DM counters in Hippowdon, Tyranitar, and Specially Defensive Heatran. Every team in DM should have a Raikou counter, but the real mission is to win the weather war. I've had many battles with guys like Nollan destroyed my entire team when I accidentally lost the weather war (In case you haven't guessed, weather is so important in DM now that Hail is viable as well.), just showing Raikou's power. The best way to beat it is to win the weather war, simple as that, or else, you have to predict like a fucking monster. Good luck!
And finally...
Flygon
Now, this mon was already good before, but now its even better. Based Dusk gave Flygon Sand Rush, and now it wrecks lives. Flygon can don a Choice Band, outspeed near everything in Sand, and hit hard with Outrage and Earthquake (and Fire Punch for Ferro, if you care to do that). He's an awesome mon to use in Sand, and even does okay outside of Sand as a good answer to threats found on Sand teams like Scizor and Ferro.
That's all I can think of as of now. There are plenty of threats left, so if any of the players can think of other ones, post them so the new players can get a sense for what's really good and what needs to be countered!