Plant had probably the absolute worst decline in voice quality I've heard. The first Zep album I bought was The Song remains The Same. I think I tried to convince myself that it was great, because Led Zep had this near mythic reputation, but I was secretly thinking "You've got to be kidding me, this guy's voice sucks" I later heard their earlier records, and Plant's voice sounded a lot better on their first two. It declined a bit on 3 to 4, but by the time of Houses Of The Holy, forget it. His voice was gone. Through overuse, surgery, age and smoking, Plants voice has been an annoying whine ever since, both on stage and record.
Coverdale's voice has fared better, although to be honest he seems to try a bit too hard at times to be "hard rock god" and can sound a bit silly at times.
Coverdale wins this one for me.