Posiflex rods flex, hence the name. They are graphite-wrapped wood, not pure carbon. Seem rather worthless to me. I had an AmPro with those and for me it was pretty much no different than any other Fender neck.
Warmoth necks have either steel or carbon fiber rods, and are good and stable. But the steel rods make them heavy.
USACG necks have carbon fiber rods and are stable while being very lightweight, a good deal lighter than Warmoth.
You can also consider quartersawn lumber for the neck shaft. It's quite stiff, resistant to bowing and weather changes.
Roasted maple is also supposed to be more stable, and I love it, but I really don't know if it's more stable or not. It just looks so great and smells great, like maple syrup (really, because the roasting cooks the sugars in the wood).
Having said all this, the only necks I have had that never had any response to weather were the Kramer aluminum neck, and the Modulus all-carbon fiber neck. The Kramer neck was very heavy, and any carbon fiber neck is expensive.
If I was ordering a new neck specifically for stability, it would be from USACG, made from quartersawn roasted maple.
Wood is a natural, organic thing that just naturally changes with the seasons. IME, no matter the rods, etc, wood will move to some degree or another. Just the way it is, and no biggie to me. Also IME the most stable neck is the one that is thick. They don't feel the best, but they are the ones with the least movement. For obvious reasons.