Which IV should I choose ??

Follow the instructions in the original IV determination post.
That means, power the pokémon up and check IV values until you get only one result left. Although that means to spend dust on a possible not-so-interesting pokémon. So choose whisely.

You could also try to fight against a known gym defender (IVs, move set and lvl known) and try all possible combinations in pokebattler and check which one reflects your fighting result the best. That method though coult turn out to be not as reliable or quick than the power up one.

Do you also use the in-game appraisal? The IV Calc will often give you some numbers that are not in the correct range, so that would also limit the range.

Place it in a gym. Scan with GymHuntr.com. Wait for gym to change. Scan again, then view gym history and click on your pokemon.

Obviously too slow to check every pokemon this way, but can work for occasion checking and better than spending stardust to try to narrow down results.