Final answer:
The letter that starts 'nothing', ends 'sun', and represents 'Number one' is most fittingly the letter 'N'.
Step-by-step explanation:
The riddle you've posed is intriguing and fun. It's asking for a letter which is the start of nothing, the end of sun, and also Number one.
Let's break it down: 'nothing' begins with the letter 'N'. The word 'sun' ends with the letter 'N'. Additionally, 'Number one' can be seen as a representation of the first position, which in the alphabet, the letter 'A' holds. However, since 'N' fits both as the start of 'nothing' and the end of 'sun', while 'A' fits only the idea of being number one, the best answer aligning with most of the clues is 'N'.