The most important is the Producers because producers are foods of the first consumer which is the food of the second consumer which is also the food of the third consumer. What we can agreed is the producer is a plant and the first consumer is a herbivore and the second and the third is a carnivore.

The least important are the Third Consumers because third consumers only is optional and the food chain can end with the second consumer.

Example of a food chain is: Grass - Rat - Snake - Hawk
The Grass is the producer, the Rat is the first consumer, the Snake is the second, and the Hawk is the third