If I want help to program an algorithm in PARI ,
which site is the proper one ?
I asked several question about tetration and power towers, and still do not have the desired algorithm managing all cases.
If it is about the algorithm itself, ask it here (or TCS if it is "research level").
If it is about how to implement the algorithm in PARI, use StackOVerflow. They even have specific tags on PARI for the C library and PARI/GP for the Computer Algebra System. (I am not sure which one you meant.)
I'd like to note that there is currently a proposal for an SE site that would take exactly this kind of question. It needs more suggested questions, so posting your question there as well would help it on its way (it won't get an answer, though - so post it on SO too as suggested above.)