Ronnie James Dio!
I agree Freddie Mercury is one of the greats--and while having a wide vocal range alone doesn't necessarily make a singer a "great" singer (whatever that definition means), it's interesting to note that Axl Rose actually covers more most other rock singers--even pop singer Mariah Carey!
Take a look at this infographic to see where your favorite stands with regard to range:
The Vocal Ranges of the Greatest Singers | ConcertHotels.com