There are several that come to mind.
Wool is one of course it is very adaptable and the sheep tend to be very good at changing grass and water into this excellent product.
Other fibres are not so well recognised are Hemp (yes THAT plant), but it can be made into quite good yarn and fabric. Kenaf is also very similar and is apparently also capable of similar use. These are like wool, quite durable and can be grown in very dense plots although they do need a good water supply so should only be planted in high rainfall areas.
Cotton, NO NOT bloody likely.. Here in Australia it is being inefficiently grown in some inland areas where the water supply is rather dodgy. As a result one of our largest rivers is not stuffed fro excessive removal of the water both directly from the streams and from the ever lowering water table. Cotton should ONLY be grown in areas where the conditions are suitable. This goes for all crops of cause. Cotton also has a huge requirement for chemicals to aid growth and pest control. I do not actually have a problem with cotton so long as it is not being grown inefficiently in “My Backyard”.
Bamboo, now here is an interesting fibre. Well actually, two fibres. There is the fibre gained by direct break down of the bamboo stem. and this is really a good strong fibre but is seldom seen in fabrics. I do not know how good it is for clothing. The other “Bamboo” fibre is actually Rayon produced from a different feed stock. Rayon was traditionally made from wood although it can be produced from almost any source of cellulose. For the bamboo sourced product the basic process is the same. The feedstock is treated with chemicals that dissolve the cellulose. For rayon this is usually hot caustic soda (Sodium Hydroxide) solution. This product is then fed through a series of fine holes into an acidic bath. This regenerates the structure into threads for spinning and weaving. The rayon industry is noted for it’s use of some rather nasty chemical but is now days also noted for it’s recycling of these reagents. There have been some very bad accidents in the past however. I have given a very brief account of this, for more on this product have a look at; Viscose - Wikipedia
Now I come to the real synthetic products. Are any of them worthy of being called eco friendly? It is very difficult to work out which if any individual fibre is better. Most are based upon petro chemical (oil) or coal by-products, The production of many of these is not necessarily bad so long as good house keeping is maintained. The big problem is that they are not readily recycled for reprocessing and if dumped they do not decompose into innocuous residue. Reprocessing might be OK were it not for the trend of mixing different types of fibre together. Sometimes you can even find Polyester and Rayon together. The term “Polyester” is a blanket term that actually covers several similar products but with different chemical structure.IFR Stage Curtain Fabric manufactures
I see some of here has suggested Leather. Now that is not such a bad idea although it does tend to be expensive now and if more people began demanding it the price would really go high.
What do I wear? Well as it is cold right now I actually have almost all of the above, good and bad, with the exceptions of Hemp/Kenaf or Bamboo/ Rayon on right now. We are somewhat at the mercy of manufacturers in what is available at a reasonable/ affordable price, so in spite of my grievance with some of these I am economically forced to go along for now.