Ah yes, card rankings. We all love looking for the best cards in the game and seeing what they are, but what defines "best". According to dictionary.com, best is defined as "adjective, superlative of good, with better as comparative. 1) of the highest quality, excellence, or standing 2) most advantageous, suitable, or desirable." Even though this may seem defined, it is still subject to varying opinion and bias. As far as I know, the only thing that has no bias: is numbers.
The card? Furious chicken. 
The furious chicken at lvl 1 is a zero mana, one speed, one health neutral card with so many uses. Currently, the furious chicken is the only no mana costing card in the game (will soon change with release of new set though). This means that as long as your lineup has space, you can use the furious chicken.
Of course this card shouldn't be used in every deck or battle, only in battles where you use up all your max mana and still have space in the lineup. For example, if you are expecting your opponent to use a card with snipe, place the chicken in the back of your lineup to tank an attack for you, saving the health of the card placed before your chicken. If you are expecting your opponent to use snipe, and you are not using any reach cards, it may be a good idea to place your chicken second in the lineup. By itself the chicken isn't too useful, tanking only one shot, but if used in combination with other cards, can make a much larger difference. Using venari wavesmith? 1 health 2 shield free chicken! Using mylor or owster? That no cost chicken can even do damage to your enemy, not just serve to block one attack!
But these reasons are still subject to bias and relativity to a specific situation, and there are multiple other cards that can be more effective while being cheap. Shadowy presence is just one mana, but has four health, thats four times as much as the furious chicken. But alas, it still costs mana, and here I will bring in the mathematical reasoning behind the chicken. Even if there was a card that cost one mana and had 10 health, the chicken would still be better. The card would have a health/mana ratio of 10/1. However the chicken...the chicken would have a ratio of 1/0. Mathematically, that gives us an answer of "undefined", but logically that just means the card is free, and who wouldn't want a free card?
Credits: screenshot from splinterlands, gif from gfycat.com https://gfycat.com/acidicbrokenamurratsnake