"My girlfriend is the most beautiful."
Just to clarify for those who don't know, The 是⋯⋯的 (shì... de) construction is used to draw attention to certain information in a sentence. It's often used to ask questions that seek specific information, or to explain a situation by emphasizing a particular detail. While not strictly tied to any "tense," the 是⋯⋯的 construction is frequently used when asking or telling details about the past.
The "的" at the end allows you to use "是", irrespective of "最" or "很".
Note that "她漂亮" and "她很漂亮" are both correct sentences, and there's no "是" in either of them, so "是" doesn't get "replaced", because adjectives are essentially stative verbs in this kind of sentence.
One thing the version without "很" can do is have a comparative implication, i.e. something like "she's the pretty one (in comparison with someone else)". A function of "很", then, can be to remove any trace this comparative implication. (For some types of adjectives, e.g. more abstract or multisyllabic ones, it's not really necessary, but this is a more advanced grammar point.)
The common use of "很" in this way is why it loses its force and often doesn't need to be translated as "very", and therefore why it's sometimes referred to as having a copular function, though I think that's questionable terminology. It shouldn't be confused with "是" or be thought of as a replacement for "是".