What is the fast approach of Pooling system in Unity? A List<Transform> , simple Transform[] array , Queue or Stack ?
You should choose the right data structure regarding your actual requirements. There is no too much difference between List<T> and T[] in C#, List<T> is actually an abstraction of the array in essence.
Ask yourself which functionality you expect from your object pool, e.g, if the number of elements is fixed you could use T[]. If the number of elements changes, you need to add\remove elements, then maybe List<T> is a better option.
Also, each collection has its own performances expressed as asymptotic complexity. You could consider that regards how many elements you do expect and which operation you are going to perform, read here.
In the end, there is no silver bullet you should choose what fits the best.