Sorry, but Ann Wilson is not underrated. She is largely viewed as one of the top rock vocalists of all time, and perhaps among the very top handful of female ones. So much so, that I think she is overrated (not that I dislike her singing).
As for underrated, I would say Ozzy Osbourne. He is generally viewed as a great frontman/personality who can't really sing. I, OTOH, think his voice was absolutely incredible early on...and got progressively worse, such that it was toast by Vol. 4, and never came back. But first album Ozzy...effing hell, man. He had some real pipes on a technical level, and knew how to do something new and interesting with them. Yet he gets practically no credit for actually being a "good" singer.
Most overrated is, IMO, Ian Gillan. And a close second would be Bruce Dickinson. Both of these singers are actually quite horrible and unskilled on a technical level. And while some technically horrible singers have good musical style and taste in spades, and because of this, can sound great anyhow, both Gillan and Dickinson have neither good musical style nor good musical taste, and thus eventually musically tanked their respective bands when they joined, or soon after (I still dig In Rock and The Number of the Beast, and some other Gillan-era Deep Purple and Dickinson-era Maiden).