PvE: Like all different non-fully-evolved Pokémon, Haunter has no place right here.
PvP: Haunter’s max CP is 1963 (even decrease with lower than good IVs), which successfully limits its viability to the Great League.
Since its solely real looking area of interest is in Great League PvP, it is best to energy it up as near 1500 as potential with out going over.
However, you also needs to take into account whether or not you could wish to commerce it sooner or later. Legacy strikes are sometimes very useful, and since buying and selling rerolls the IVs that would probably put it over the 1500 threshold should you energy it up an excessive amount of, particularly for such an previous Pokémon that has a excessive probability of triggering a fortunate commerce. If you haven’t any precise plans to make use of it, take into account leaving it as is as a substitute.