In my observation, the most commonly mispronounced English letter is "N". Instead of [ɛn], many Chinese pronounce it like a prolonged [n], missing the vowel [ɛ], or the vowel is too short and weak to be heard. This is particularly evident when he/she spells a word on the phone, where a very clear, distinct pronunciation of each letter is expected, e.g., "His name is Wang, W-A-N-G" (['dabulju:], [eɪ], [n], [dʒi:]). Due to the nasal nature of [n], with the initial vowel [ɛ] stripped off, the other side of the telephone would likely ask "W-A-What-G?" Of course giving an example word solves the problem, such as "N like in Nancy".
Note the IPA symbols for the mispronounced letter "W" in the above example sentence. The first vowel I wrote is [a] and the second [u], while the correct pronunciation of "W" is ['dʌbə(l)ju:] (parentheses for the optional phoneme). Too many Chinese pronounce [ʌ] as [a], because [ʌ] does not exist in Chinese. This substitution also occurs for [ɑ], which again does not exist in Chinese. So letter "R" is read like [ar] instead of [ɑr]. Fortunately, [a] alone does not occur in English; it only exists in diphthongs (e.g. [ai]). Mispronunciation by substitution of it for [ʌ] or [ɑ] won't cause misunderstanding, just a distinct foreign accent.
I'm not sure why "N" is read by Chinese as [n]. If you know why, I'd like to hear your comments.
Note: The difference between [ʌ] or [ɑ] and [a] is clearly indicated in the International Phonetic Alphabet vowel chart. Both vowels not existing in Chinese are pronounced in the back side of the mouth.