Mutawaatir refers to Hadith narrated by such a large number of narrators that it is impossible they could have agreed upon a lie. However, the question seems to ask about the most authentic Hadith collection. Hadith reported in both Sahih Bukhari and Sahih Muslim are considered the most authentic (called Muttafaqun Alayh - agreed upon). These two collections are the highest authority in Hadith literature after the Quran.