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I don't know that there is universal agreement on which reference grammar is the "best", much less universal agreement on what criteria one would use to evaluate reference grammars. You might consider doing a web search on "spanish reference grammar in English", see what hits you get, and look for customer reviews.
The reference grammar in English that I am most familiar with is "A New Reference Grammar of Modern Spanish" by John Butt, Carmen Benjamin, and Antonia Moreira Rodríguez. I bought the 5th edition many years ago, a few years before the 6th edition (the most recent) was published. I like it a lot.
For grammar information in Spanish, the most authoritative is probably the Nueva gramática de la lengua española (abbreviated as NGLE prepared and published by the Real Academia Española (RAE). Their website supports looking up both grammar information and dictionary entries from their published works, including both the Diccionario de la Real Academia Española (the DRAE and the Diccionario panhispánico de dudas (the DPD).
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