Which one do you want to be the sole survivor?
Plants (land only)
Birds (non-flying included)
Animals (land only, non-bird)
Marine life (must live in water, all kinds)
Insects are intentionally excluded.
If needed, the last "life-form" would develop ways to sustain itself, including eating each other.
If the options are confusing, go with your gut. Examples: horse is a land animal; penguin is a bird; apple tree is a plant.
ETA: Sustaining itself includes its respiratory ability - it would evolve or adapt as needed, too.
I got an error message when posting this and didn't realize this entry went through.