If you were to overly simplify it, Pokemon is just Rock-Paper-Scissors. Fire beats grass, grass beats water, and water beats fire. But what makes this so much more interesting is that there are currently 18 different types that can be combined with another. This means that there are 171 unique type combinations in Pokemon. That's huge!
However, not all dual-types are created equal. The best have been broken down for you in this list. If you decide to put any Pokemon with these typings on your team, you won't be disappointed.
3 Offensive
In order to win Pokemon battles, you're going to have to exploit your opponent's weaknesses and hit them for Super Effective attacks while also getting a same type attack bonus or STAB. These Pokemon are your best bet at doing that to as many Pokemon as possible.
Ground/Flying
You'd think that the Flying and Ground types wouldn't work well together since they're literal opposites of each other, but they actually synergize well since defensively they cover a lot of each other's weaknesses. More importantly, they make for the third most powerful attacking combination in the series. While the Gliscor line shows Grond/Flying's defensive prowess, both Landorus forms represent the true offensive destruction of the typing. They've been dominating competitive Pokemon in both singles and doubles for a reason!
Fighting/Ice
Since the Fighting/Ice dual-typing is only represented by the mediocre Crabominable, it's easy to forget how good it really is. Fighting/Ice is the second best attacking combo in the game, hitting 51 percent of all Pokemon forms for STAB Super Effective damage.
It's just a shame that Crabominable is the only Fighting/Ice-type. It has an impressive attack stat, but its Speed and defenses are massively lacking. Hopefully, we'll get a speedy option soon!
Ice/Ground
It's appropriate that the only fully evolved Ice/Ground-type is Mamoswine because its attacking potential is truly mammoth. Ice/Ground is the most powerful dual-typing since it can hit 53 percent of all Pokemon forms for STAB Super Effective damage. Mamoswine makes it even better with its 130 base Attack stat. Just slap a Choice Band on it and let it wreak havoc! It may be simple with no real tricks up its sleeves, but Ice/Ground is by far the best attacking dual-type.
2 Defensive
Hitting hard is great, but it's not worth anything if your team can't stick around to hit at all. These dual-types represent some of the best defensive Pokemon in the entire game.
Electric/Steel
It can be very easy to dismiss the Electric/Steel-typing since it can be taken out by a well placed Earthquake. But what players will be missing out on is a Pokemon with 11 resistances, more than any other 'mon out there. And the Magnezone line is able to take advantage of this with incredibly high defense that makes them last forever while dishing out powerful special attacks.
Grass/Steel And Bug/Steel
These two had to be included together because of their similarities. Both Grass/Steel and Bug/Steel have a glaring weakness to Fire-types. However, you cannot ignore that both of these dual-typings with their long list of resistances and short list of weaknesses have some of the best defensive Pokemon ever created: Ferrothorn and Forretress. Not only can they take their respective hits, but they support their teammates with entry hazards and status effects.
Even a more offensive oriented Pokemon like Scizor is made better since its excellent typing gives it more time to set up Swords Dance. Bug/Steel is so good that it almost makes Wormadam viable. Almost.
Steel/Flying
Clearly, Steel is the best defensive typing out there with ten resistances and an immunity. However, it is weak to three of the more common attacking types. So why not get rid of two of them? Adding on the flying-type makes the Pokemon take neutral damage from Fighting-types and completely immune to Ground-type attacks at the cost of a weakness to Electric attacks and two less resistances. This is absolutely worth it and makes Skarmory, Corviknight, and Celesteela stand out as defensive behemoths, even if it's only in competitive singles.
Ghost/Dark And Ghost/Normal
See? Not every defensive entry is Steel-type! Instead of having a ton of resistances, Ghost/Dark and Ghost/Normal have the most immunities of any type while also having only one weakness. In fact, Ghost/Dark only got its weakness back in gen 6 with the introduction of the Fairy-types. Whether you choose Sableye, Spiritomb, or Hisuian Zoroark, you'll be utilizing some of the most irritating Pokemon in the entire game.
Steel/Ghost
From this list, you can surely tell that the two dominant defensive types are Steel for its resistances and Ghost for its immunities. So why not have the best of both worlds? Combining Ghost and Steel gives the Pokemon a whopping nine resistances and three immunities. Aegislash is the perfect Pokemon to represent this typing since its Stance Change ablity allows it to really take advantage of that typing, especially in its shield form.
1 Miscellaneous
While not necessarily the best attackers or defenders on paper, these types are still incredibly effective and a worthy addition to any team.
Water/Ground
Water/Ground is a great defensive typing that takes away Water's Electric weakness and trades it for a double Grass weakness, but since Grass-type attacks are much less common this is a welcome swap. Plus, most of the Pokemon that are Water/Ground have either Storm Drain or Water Absorb, making them immune to Water-type attacks. Just give these Pokemon an Ice-type attack and you'll have a teammate that will always be reliable, especially in the Hoenn region.
Fire/Steel
Other Pokemon may have the same amount or more resistances than Fire/Steel, but no other typing has more double resistances. Fire/Steel-type Pokemon will take just one fourth damage from five different types including the common Ice and Fairy-type attacks. Plus, it resists four other types and is immune to two since the only Fire/Steel-type Pokemon, Heatran, has the ability Flash Fire. It's tough to get Heatran, but it is a worthy addition to any team.
Dragon/Ground
On paper, Dragon/Ground doesn't look like the most effective typing since it only hits 32 percent of Pokemon Super Effectively. But what makes Pokemon like Garchomp so devestating is that Dragon/Ground can hit almost every Pokemon for neutral damage, except for Steel/Flying-types.
However, all three of them can easily be handled by teaching Garchomp Fire Blast. Basically, Pokemon like Garchomp and (to a lesser extent) Flygon can handle anything you throw at them.
Fire/Ground - But Only Primal Groudon
The first Fire/Ground-type Pokemon that comes to mind is always Camerupt who isn't that great. Even though it can hit 47 percent of all Pokemon for STAB Super Effective damage, it has a glaring double weakness to Water-type attacks which is the most common type in the game. However, Primal Groudon has the same typing and is able to get away from that because of its Desolate Land ability which nulifies all Water-type attacks. This means Primal Groudon only has one weakness and can eviscerate any opponent that comes its way. This is why Primal Groudon is truly legendary.