From my notes (a text on APUG I posted some time ago, shortened version below):
Glassine products: generally very good. There can be problems with "seems" in conditions of high humidity and when pressure is applied to the sheets. Otherwise give good protection for decades. 135 stripes are never in touch with the seems. Not really "museal" though. Glassine is not compliant with ANSI IT9.2 - 1991. If you want something really good, you should look for something complying with that norm. Current version ANSI IT9.16 actually.
Uncoated transparent polyester (such as Mylar) is probably the best, but it's not the cheapest, as you'd guess.
Uncoated polypropylene such as ClearFile is also good.
High-density polyethylene is also "probably" good and it is one of the cheapest solutions. (Not convinced about that, see prices below).
ALL those plastic solutions struggle with static electricity problems (they attract dust) whereas paper-based product (such as glassine) do not. Besides, you can write with a pencil or a pen on a paper or glassine sheet.
Anything containing PVC or chloride compounds must be avoided.
Low-density polyethylene is also to be avoided.
From what I gather, the devil is in the details, any coating or glue has the potential for damaging film, more than the material itself are the "accessory" substances that cause problems.
Sometimes the material is certified as being standard-X compliant, but the final product (which includes coatings and glues) is not. Watch for the certification of the photographic sheets, not the certification of the material.
I am satisfied with my glassine sheets. I only do 135 and don't have any humidity problem in my house. I like being able to write on the sheet and I like not being bother by static electricity.
Anyway I've seen archival polyethylene satisfying the PAT test (Photographic Activity Test ISO 14523 e ISO 18916 Image Permanence Institute, RIT, Rochester NY), 100 sheets for €30,00. Not the cheapest, but acceptable at 30 €cents per sheet.
ClearFile are polypropylene and also satisfy PAT test, seen 100 sheets for €24,00.
I have no idea about "Mylar" products availability and cost. Besides those, polyethylene or polypropylene products should be good provided they have some certification regarding the entire photographic product.