It is subjective to determine which capital English letter would be the best fighter based on its physical shape alone. However, one might consider the letter X as a good fighter due to its symmetrical, rigid, and angular structure, giving it the appearance of a strong, stable, and agile figure.

It is also subjective to determine the worst letter, but one might consider the letter O as the weakest fighter based on its physical shape alone, due to its round and simple shape, with no edges or angles. This might give it the appearance of being less agile, stable, and strong compared to other letters.