What's just as important in a Pokemon battle as executing attacks is a Pokemon's ability to stay on the field. There are three stats in Pokemon's stat system concerned with that: HP, Defense, and Special Defense.
This list will look at physical Defense, and clearly, some types fare better than others in that stat. One way to compare the types' defensive abilities is to pit their best against each other. Every entry on this list will do just that: list the respective type's Pokemon with the best Defense stat and see how it stacks up.
Note: This list will be ranked without taking Mega Evolutions into account, though they will be discussed with every entry. Arceus holding a type plate is ineligible.
18 Dark: Scrafty
Put a hand up if you predicted the Dark-type to be last place. While Scrafty's 115 Defense isn't at all bad, it's pathetic when compared to the other Pokemon here. That's not to say the Dark-type is bad defensively, but no Dark-type Pokemon has an absurdly good Defense stat, with the next best being a three-way tie between Tyranitar, Umbreon, and Drapion (110).
That's without considering Mega Evolutions, though, as Mega Tyranitar (150) and Mega Sableye (125) beat Scrafty in that stat.
17 Normal: Arceus
While Arceus holding a type plate is ineligible for every other type, there's no choice but to put Arceus as #1 among the Normal-types. It's at 120, but all of its stats are at 120 each. That said, it makes sense to not give a type with no resistances a defensive specialist.
When including Mega Evolutions, Mega Audino takes the honor at 126. After those two, the only real notables are Regigigas (110) and Miltank (105).
16 Electric: Alolan Golem
The Electric-type is certainly not known for defense, and that shows with Alolan Golem being the only Pokemon to reach the 130 mark on the spot. The Electric-type is so subpar defensively compared to other types that its best representative is an offshoot regional variant.
After Alolan Golem, the next best are Zekrom (120), Alolan Graveler and Magnezone (115), and the Rotom appliances (107).
15 Grass: Ferrothorn & Kartana
There's an interesting dichotomy between these two Pokemon. While they couldn't be more different in how they function in a battle, Ferrothorn and Kartana are both Grass/Steel-types with 131 Defense.
Among Steel-types, that's nowhere close to the best, but it's good enough to be the best among Grass-types, with Leafeon (130), Tangrowth (125), Mega Venusaur (123), Chesnaught (122), and Gourgeist (122) as the next contenders.
14 Dragon: Turtonator
While there are a lot of strong Dragon-types, none of them really specialize in Defense. The one here that does is the non-Legendary, competitively-mediocre Turtonator, coming in at 135.
Mega Salamence is second place at 130, and Kommo-o is second place among non-Megas at 125. Those three are followed by Zygarde 50% and Complete (121) and a four-way tie at 120 between Mega Latias, Dialga, Giratina, and Zekrom.
13 Flying: Skarmory
The Flying-type isn't terrible on the physically defensive end, but much like the Dragon-type, there aren't really any absolute specialists other than the representative for the entry, the 140 Defense Skarmory.
Lugia is a relatively strong runner-up at 130, tying with the aforementioned Mega Salamence. Gliscor (125), Mega Pinsir (120), Rotom-Fan (107), and a three-way tie (Gligar, Mandibuzz, Corviknight) fly right below them at 105.
12 Fire: Torkoal
Torkoal also comes in with 140 Defense, topping off the Fire-type pool in that stat. It ranks just above Skarmory on this list because it faces a bit more competition.
The Dragon-type entry, Turtonator, comes in second place with its 135. Then, there's a three-way tie at 120 (Magcargo, Volcanion, Coalossal), followed by Mega Charizard X (111), Alolan Marowak (107), and Heatran (106).
11 Fighting: Crowned Zamazenta
It's a close fight between Crowned Zamazenta and Buzzwole for the best defensive Fighting-type, but Crowned Zamazenta's 145 just edges out over Buzzwole's 139. Regardless, you can rely on both to take a few physical hits.
The rest of the Fighting-type pool is decent when it comes to Defense, with Cobalion (129), Kommo-o (125), and Chesnaught (122) making up the rest of a solid top five.
10 Ghost: Doublade
Thanks to its nerf in Generation VIII, Aegislash (140 in Shield Forme) actually has less Defense than Doublade (150). That's a shame, as it would have tied with its pre-evolution for the best Defense for a Ghost-type.
Alas, it has to settle for third place, with the Yamask-line cousins Cofagrigus and Runerigus sharing second (145). After them, you have two more evolutionary mates in Dusknoir (135) and Dusclops (130), followed by Mega Sableye (125), Gourgeist (122), and Giratina (120).
9 Fairy: Carbink & Diancie
After showcasing three nepotistic evolutionary pairs in the previous entry, here's a pair of Pokemon that look like they come from the same evolutionary family. Diancie and Diancie-Lite, a.k.a. Carbink, have nearly the exact same base stat total aside from Attack and Special Attack, with their Defense stats reaching 150.
After that is quite a fall-off, with Mega Audino (126), Mega Mawile (125), and Galarian Weezing (120) being the only ones left to reach the 120 mark.
8 Poison: Toxapex
Speaking of Weezing, both its regional variants sit comfortably at third place for the best Poison-type in terms of Defense (120). The two Pokemon above it are Mega Venusaur (123) and the runaway winner, Toxapex (152).
The only other Poison-type that even reaches 100 in that stat is Drapion at 110. It's an underwhelming pool, to say the least.
7 Psychic: Deoxys-Defense
There are quite a few Psychic-types with very good Defense stats. The leader of that pack is Deoxys-Defense at 160, but it's by no means a drastic dropoff after that.
In fact, when factoring in Mega Evolutions, Mega Slowbro has it beat at 180. Then, Ice Rider Calyrex and Mega Metagross tie for third (150). Among the rest of the Pokemon that surpass 120 Defense, you have Cosmoem (131), a three-way tie at 130 (Lugia, regular Metagross, Uxie), Dusk Mane Necrozma (127), Mega Latias (120), and Cresselia (120).
6 Water: Cloyster
When looking at Mega Slowbro's other type, Water, it ties with Cloyster for best Defense at 180.
While it's a big drop after that, there's still a great pool of defensive Water-types, like Toxapex (152), Golisopod (140), Carracosta (133), Pyukumuku (130), Relicanth (130), Omastar (125), Mega Blastoise (120), and Volcanion (120). This group is nothing special when compared to the others, but is, if nothing else, relatively solid.
5 Ice: Avalugg
Cloyster comes close to getting the spot here for Ice-types as well, but Avalugg just ekes out an extra 4 Defense to be the breadwinner. For a type that's considered to be very weak defensively, 180 and 184 respectively are not Defense stats to scoff at.
The rest of the pool is less than stellar, though. While Ice Rider Calyrex is excellent at 150, there are only two other Pokemon that reach or surpass 120, being Glastrier (130) and Alolan Sandslash (120).
4 Ground: Steelix
This point of the list goes past the 200 mark now, with Steelix reaching that number on the dot and taking this spot for the Ground-types. Its Mega Evolution is even stronger on that end, at 230.
Its pre-evolution Onix ties with Primal Groudon for second place (160). From there, you have a very solid line of defensive Ground-types: Runerigus (145), regular Groudon (140), Golem (130), and Rhyperior (130).
3 Steel: Stakataka
As previously mentioned, Mega Steelix has 230 Defense, which would have tied it with Mega Aggron for best among Steel-types. Otherwise, fellow Steel/Rock-type Stakataka takes the cake at 211.
Regular Steelix (200) and Aggron (180) come next, followed by Bastiodon (168), Mega Metagross (150), Registeel (150), and Doublade (150). Many more reach 140, confirming the Steel-type's supposed defensive prowess.
2 Bug: Shuckle
When including Mega Evolutions, Mega Aggron and Steelix tie with this Pokemon for the best Defense out of any Pokemon. Otherwise, when looking at all Pokemon pure and unboosted, that honor, with 230 Defense, very stupidly goes to Shuckle.
Second place is obviously nowhere close, but the Bug-types have a surprising amount of defensive capabilities. Said runner-up Pokemon are a three-way between Scizor, Forretress, and Golisopod (140), with Buzzwole (139) creating an excellent top five.
1 Rock: Shuckle
Shuckle's competition from the Rock-types is much more formidable, with two other Pokemon reaching 200: the aforementioned Stakataka (211), and Regirock (200).
From there, a bunch of old friends make appearances above the 150 threshold, namely: Aggron (180), Bastiodon (168), Onix (160), Mega Tyranitar (150), Carbink (150), and Diancie (150).