For accurate load flow calculations on large power systems, the best method is

N-R method as it is more faster, more accurate and more reliable than others.

(For your info) Advantages of N-R method:
1. Number of iterations are less, so that it has fast convergence.
2. Convergence is not effected by the choice of slack bus.
3. No need of accelerating factor.
4. Applicable for large power system network.
Disadvantages of N-R method:
1. Time taken for each iteration is larger if size of the Jacobian matrix is larger.
2. Computer memory required is larger.
3. Computer programming is difficult.