I've always considered Overkill to be just outside "The Big Four" - a second tier metal band along the lines of Testament. When I was growing up, I did not like Overkill at all. It wasn't until Horrorscope that I changed my mind, but it still took some time for me to really warm up to them. I credit Killbox 13 for turning me into a full on fan, but it's the album that came out right before that one, Bloodletting, that I still consider to be their best.
Of The Big Four, I think Megadeth is most like Overkill, and until the 90s, Megadeth was the superior band, but in the years since 1999 (Risk), I would say that Overkill is now the better band.