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Best defensive types and offensive coverage per generation:
>Gen 1
>Defense
Dragon, Steel, Grass, Ground
>Coverage
Ground, Grass, Bug (these 3 cover all 8 types)
>Gen 2
>Defense
Steel, Fire, Ghost, Fairy
>Coverage
Fighting, Flying, Ice (these 3 cover all but Ghost)
>Gen 3
>Defense
Steel, Dragon, Ghost
>Coverage
Ground, Electric, Fighting (these 3 cover all but Fighting and Psychic)
>Gen 4
>Defense
Steel, Dragon or Grass, Water
>Coverage
Ground, Fire, Flying (these 3 cover all but Ghost)
>Gen 6
>Defense
Steel, Poison, Ground
>Offense
Flying, Steel, Ground (these 3 cover all but Psychic)
>Gen 9
>Defense
Steel, Dragon, Dark
>Offense
Fire, Dark (these 2 cover all but Electric, Normal and Water)
5/7/8 all have more Gym Leaders than there are Mighty Trainers so you might be preparing for a type that you won't ever fight
Steel is predictably the general best defensive type while Ground offers great coverage in most gens.
Gen 1's leaders are the easiest to counter, Gen 6 is probably 2nd easiest, while 2 and 4 are the hardest of the Guaranteed 8 Leaders.
