Best filament to use in high humidity environment?
You don't provide information about the part, but in general PETG, ABS, ABS plus/Pro and ASA are not going to have any issue with humidity for a long time. If it's outdoor, avoid ABS and use PETG or ASA.
What is important is to keep the filament dry BEFORE printing. After printing, no problem. In fact, nylon after printing can be wet on purpose to significantly increase impact strength (and reduce tensile strength).
PLA in high humidity environment may lose some of its strength, but there are PLA parts under the rain which hold perfectly for long time, if they are not loaded significantly.