First of all, it is questionable as to whether one can have stages of perfection. How would "most perfect" work if another plan were "perfect"?Originally Posted by toannt82
That aside, #2 is the correct choice. If one uses "most" there must be other plans. The definite article is used for a specific plan, the one judged to be the best.
We could say "a perfect plan", because that means any plan that fits the purpose and is judged to be "perfect".
You will hear #1, but I would consider it informal at best.
The Constitution of the US contains the phrase "to form a more perfect union". The indefinite article is OK there, because there are potentially many unions that are more perfect than the original. Had Jefferson said "most perfect", he would have used "the".