Polyethylene, either high or low density. HDPE is high density polyethylene. After I typed that I realized more of my squeeze bottles are LDPE, but it was the PE part that was relevant. Polypropylene, aka: PP, would be better as it is even more resistant to more chemicals, but I've never found drip bottles made out of it.
I like glass too, but I give too much juice away to convert completely. If I have a plastic bottle of something someone likes, then I don't feel any pain in my wallet giving them the bottle. (And if someone wants to return the favor with empty Halo bottles, I get very generous...)
I've been doing this so long that I remember when TPA/TFA use to ship the glass bottles with drippers. Man, those were the days. But I still use old flavoring bottles from her with the drippers that came with them.