a = input("输入一段文本")
s = list (0 for i in range(26))
for i in a:
    b = i.upper()
    if(b<='Z' and b>='A'):
        s[ord(b)-ord('A')]+=1
f =chr(ord( 'A')+s.index(max(s)))
print(f)

示例：

输入一段文本ASDFHJDFJFEIWEQQAASSAAA
A

有帮助望采纳

-- coding:utf-8 -

import re

def max_word(msg):
word_dict = {}
reg = re.compile("[a-zA-Z]")
word_list = reg.findall(msg)
for i in word_list:
word_dict.setdefault(i,0)
word_dict[i] += 1
max_value = max(word_dict.values())
for k,v in word_dict.items():
if v == max_value:
print ('出现最多的字母:%s,出现次数:%s'%(k,v))

if name == 'main':
msg = input('输入文本:')
max_word(msg)

#运行结果
#输入文本:abcd你好，如有帮帮助请采纳dddddddaaa,1122334AAAABBA
#出现最多的字母:d,出现次数:8