Most know body filler as Bondo. I will try to stay thorough and refer to it as filler, as that is what it is. Does that mean Bondo is what we should use? No. I've never had any luck with Bondo. Rage (brand) is what I use. I get this at my local auto paint store, and it runs me about $30/gallon, but it's well worth it. I've used various other brands, that have worked well, but the Rage seems to be the most consistent and smooth.
Now, which Rage is right for you? It all depends on the thickness.
Rage Gold is very thick and doesn't spread out too well. It is used when an area is extremely low and fiberglass can't be added to raise it. In one wipe it may add up to 3-4 wipes of what you would consider regular thickness filler.
Rage Extreme is my primary filler. I use this stuff everyday and it is the best in terms of thickness and spreading ability. It is very easy to feather edge with but can be used to build up an area at the same time, with ease.
Another good thing about the Evercoat Rage is you can mix it with the Rage Extreme filler to make a thin but buildable filler that is super easy to sand.