One pretty important thing is missing from this thread though

The C rating by itself is not an indicator of how well the battery will perform in a specific application. You multiple the C rating by the packs amp capacity to get the amperage it can deliver.

Thus, a 20c 5000 mAh battery can deliver 100 amps. A 25c 4000 mAh will also deliver 100 amps. As would a 30c 3300 mAh battery...

The C rating can be used as a general guide to the cell's quality, but you can't say "a 20c pack is fine for that". A 20c 6000mAh will deliver substantially better than a 20c 3000mAh for example - up to twice the amps.