Is it reasonable to say: "This is going to be the most amazing day by so far!" ? I mean the "by so far" part. English is not my first language, but I caught myself typing so and started wondering where did I get that from. It's just without the "by" I feel like something is missing.
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No. You're conflating two idioms with far:
Since this is a superlative expression, you can use by far; since it means up until now, you could also use so far. Either one works.
But you can't use them together. Pick one.
The most amazing day so far means 'up to now'; the most amazing day by far means 'much the best'; your example is ambiguous. I wouldn't say it's ungrammatical (by such a long way, meaning the same, would be a weak phrase, but certainly not wrong), but it's not good.