Question

What are the most important resistances in game by generation?

Comments

You mean type resistances  (Steel resisting Dark and Ghost) or do you mean something else?

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If you're asking about resistances in competitive battles, then it depends on the format as well as the generation.

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Honestly, if this question's asking about type matchups, I think it's too ambiguous to answer; the wording makes me think that the OP wants an analysis like "grass may have the most weaknesses of any type, but it is also one of the only 3 types with a ground resistance, so it's a good type" or something like that, rather than just which type has the most resistances, and I feel like trying to find the "most important resistances" like this will branch out into an open discussion without a definitive answer, which is against the rules.
I'll hold off on flagging the question until some clarification is given, though.

Answer 1

Generation 1 = psychic resistance (psychic) and normal resistance (rock). A few examples: Rhydon, Golem, Alakazam, Starmie, Exeggutor.
Generation 2 = electric resistance/immunity (ground, grass, dragon), water resistance/immunity (grass, dragon, water, water absorb), ground resistance/immunity (flying, grass, bug), normal resistance (rock, steel). A few examples: Celebi, Skarmory.
Generation 3 = water resistance/immunity (grass, dragon, water, water absorb), dragon resistance (steel). A few examples: Jirachi, Breeloom.
Generation 4 = rock resistance (fighting, ground, steel), dragon resistance (steel). A few examples: Scizor, Garchomp.
Generation 5 = fighting resistance/immunity (flying, poison, ghost, psychic). A few examples: Jellicent, Gengar, Mew.
Generation 6 = fairy (steel, poison), dark (fighting, dark, fairy). An example: Bisharp.
Generation 7 = fairy (steel, poison). A few examples: Celesteela, Bisharp, Venusaur.
The types in () are the types that resist the other type. A few Pokemon will be shown as an example (which resist one of those types shown above).
Hope this helps!

Comments

Water absorb doesn’t exist in gen 2......, other than that, good :)