I don't think Maynard is a bad singer... but he is pretty damn average. By far the weakest link in Tool's music.
Paul Stanley's vocals worked for KISS' music but I never really rated him as a vocalist (or anyone from KISS for that matter, or KISS entirely).
Tarja... Floor kills her original renditions most of the time unless Tarja goes super operatic. For example, Floor blows her out of the water on this particular song, even though she only sang it once and we have one bootleg YouTube video of it. All jokes aside regarding #AntiTarjaBrigade, she's still 2nd best here.
Ripper isn't that good, but also not so bad. Needs someone to write all the vocal lines for him and prevent him from going overboard on overdubbing. And he's a very boring frontman. That debut CWOTD album is his best performance alongside Jugulator and The Glorious Burden.