let seed = 'baa##a11111111^^11bbb';

function first_max_occurring_character_in_string(text) {
    let m = text.match(/[a-zA-Z0-9]/gi),
        c = m.map(s => m.filter(y => y === s).length);
    return m[c.indexOf(Math.max(...c))];
}

console.log(
    first_max_occurring_character_in_string(seed)
);