Busiest letter

A username analyzer reports each handle's 'busiest letter'. Given a handle containing at least one ASCII letter (plus possibly digits, underscores, or mixed case), return the letter that occurs most often, comparing case-insensitively and ignoring non-letters. Return it as a lowercase one-character string; break ties by choosing the alphabetically smallest letter. For example, 'Mississippi' returns 'i' (i and s both appear four times).