There's no "best" wood for doors. OTOH, except for balsa wood, there's nothing softer than poplar and fir is not far up that ladder. They are both really, really soft, if you're worried about dings. Poplar mills very nicely.

You want durable? Get oak.

These are interior doors, right? You have air conditioning, right? Humidity is no more an issue there than any number of places, including the entire eastern seaboard.

Again, no "best" wood.

My house is 70 years old and I live in an area with a LOT of humidity! Obviously, the house didn't have AC for decades. All my doors are solid wood and none has warped. I assume that the woodwork and doors are all pine, but since they are painted, I really have no true idea.

https://www.homedepot.com/b/Doors-Windows-Interior-Closet-Doors-Slab-Doors/Primed/Solid/N-5yc1vZc5ieZ1z0scrtZ1z0sfky?storeSelection=276,8444,6975,268,6373

Look at this link.

Solid Core Primed MDF doors. They won't warp and are available in lots of styles. You don't want to deal with grain if you paint them.

Hard, paintable solid wood is maple. Oak looks horrible when painted. Maple is stable and smooth, no grain showing when painted. However, almost everything mentioned above is less expensive than maple.

Maple, but would be a shame to paint such a nice wood. Poplar is wonderful to paint but not hard. Don’t paint Oak due to the grain.

MDF would be best.

The OP wants solid wood---MDF is not wood. MDF could be safely used as the panels, but not the frame.

"The fir and poplar will cost roughly $505 a door and I have to replace 15 doors."

Those doors are not solid wood, they are a wood veneer.

Hi Melissa, I came across your post as we are looking for wood doors for the same reason, to avoid MDF. Would like to know what you went with finally.