I've done lots of such experiments.
I'd give cattail fluff about a 7 out of 10 for ease to ignite with a spark from a ferrociom rod. However, the way it burns is suboptimal. It flares up hot and fast, and burns out very quickly. Only the outer parts burn, leaving lots of uncombusted inner material, which is surpsrisingly hard to re-ignite (from a spark).
Dried pine needles are surpisingly hard to ignite. I'd give them about a 2 out of 10 for ease of ignition. The sparks from a ferro rod bounce right off of the needles. The only way I've been able to light them with a ferro rod is by gently shaving a pile of ferrociom off of the rod, then igniting that with sparks, to create starting flames, which will be in contact with the pine needles. Pine needles also tend to burn out on their own, unless the flame is large and hot.
Foxtail gets about an 8 out of 10 for ease of ignition. It also flares up, but a little less hot, and lasting a little longer. It also burns only the outer parts, leaving the seeds, which are then difficult to ignite, and don't stay lit.
The easiest outdoor material to ignite that I've found outdoors is the large, fluffy ends of pampas grass (an invasive, non-native species to my area). These light extraordinarily easily--much more easily than cotton balls. They usually take three or fewer strokes of the ferro rod to get lit. They also provide huge aounts of combustible material. And they burn very well, creating foot-tall flames. A handful-sized clump will burn for 15-30 seconds. It burns rather thoroughly, leaving mostly ash, with just a few strands (especially is you poke it with a stick, to keep the non-combusted part on top--exposed to oxygen).
The animal fats can make decent accelerants. I've found chicken fat-saturated materials easier to ignite and longer burning than materials with other animal fats. I would be disinclined to use animal fats this way--the smell will attract predatory animals, and the rancid smell can be strong and gross.
The two best vegetable oils I've found, both in terms of how easily material soaked with them will ignite, and how long it will burn, are almond oil and olive oil.
Essential oils (like the kind you use to fragrance massage oil or incense) make extremely good accelerants. They do not make material much harder at all to ignite, and they burn hot and last a while.
Beeswax seems to burn a little hotter and last about twice as long as any other kind of wax I've tried.
Lots of first aid type items burn well. Triple antibiotic ointment burns well, since it is in a petroleum jelly base. The same is true for lidocaine gel. Most sunscreens burn well. So does chapstick. Dr Bronner's soap, however, appears to be almost totally fireproof.
