This is the biggest mushroom which I have ever seen.
"Which" or "that"? Which one is grammatically correct in this sentence?
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Grammatically, either can be used.
Stylistically, I follow Fowler's recommendation to use "which" to introduce parenthetical clauses and, when grammatically possible, "that" to introduce restrictive clauses. So I recommend "that" in your example because the clause is restrictive: you are specifying a subset of mushrooms, namely those mushrooms you yourself have seen.
But style is largely a matter of personal opinion.