While *Roots* was a massive success as a miniseries, it's important to note that it's not officially recognized as the "highest-rated television miniseries of all time". There's no single, definitive list for this type of ranking, and ratings vary depending on how they're measured (e.g., viewership, critical acclaim).
However, *Roots* was undeniably a cultural phenomenon and remains one of the most significant and influential miniseries ever produced.