As far as I am concerned, every vessel that can be cleaned and sealed is a potential fermenter, which makes the exercise difficult at best. I use:
One Coopers 30litre fermenter;
Two Bunnings 25litre water drums;
Three Decor 8.5litre food storers;
When pressed for space, I have as backup:
Four 15litre jerries;
Four 6litre jerries.
I am not fond of using the 15litre jobbies, but the 6litre ones are really useful if I screw up and achieve heaps higher extraction (and therefore volume) with a brew or if it goes in the other direction with a tiny batch. The pumpkin ale (for example) was fermented in a 6litre jerry from memory.