In this task, you are given a string with unique characters in it and you need to return the character from the string which has the maximum ASCII value. ASCII stands for American Standard Code For Information Interchange and It assigns a unique number to each character. The characters [a - z] have an ASCII range of 97-122 and [A-Z] have an ASCII range of 65-90 respectively.

Example Input: JgzvxlOntCwP
Example Output: z

Example Input: tMXbBPrUZGSWjmaxQT
Example Output: x

Example Input: wAevZUnsqQMRgB
Example Output:
w