Siebenhundertsiebenundsiebzigtausendsiebenhundertsiebenundsiebzig (seven hundred seventy-seven thousand seven hundred seventy-seven) is sometimes cited as the longest "regular" (non-compounded, non-made up) word in the German language. It is in this position because all cardinal numbers below one million are written as one word in this language. The word has 65 letters, and, although it is pretty much true, as Wikipedia states, that no German numeral has more letters if written out (with one technical exception, see below), it is worth mentioning that 777,777 is just one of several cardinal numbers with 65 letters.
Just as in English, "sieben" (seven) is the only one-digit number with two syllables in German (all the others, from one to nine, have one syllable). And just as in English, "siebenhundert" (seven hundred) is the numeral with the most syllables among the hundreds. But the tenfold numbers are different - here the number "siebzig" (seventy) has only two syllables in German, just as the numbers "zwanzig" (twenty) to "neunzig" (ninety) - and not just that, the cardinal numbers of 20, 30, 40, 50, 60, 70, 80, and 90 also all have the same number of letters in German - seven.
In effect, it does not matter which digit you put in the bolded positions: 777,777 - as long as it is between two and nine, the cardinal number will have 65 letters in German. Therefore, there are - if I'm doing the math right - in total 64 numbers that have exactly 65 letters - from 727,727 (siebenhundertsiebenundzwanzigtausendsiebenhundertsiebenundzwanzig) up to 797,797 (siebenhundertsiebenundneunzigtausendsiebenhundertsiebenundneunzig).
Therefore, 777,777 is arguably not as unique as it's made out to be - it's just the first number people seem to think of, and has the bonus of being a Schnapszahl. Last but not least - the technical exception: In Swiss German, the letter "ß" does not exist and is replaced with double-"s", adding one more letter to the word "dreißig" (thirty). This makes the Swiss 737,737 (siebenhundertsiebenundreissigtausendsiebenhundertsiebenunddreissig) the true champion - with 67 letters. Take that, 777,777!