Yes, the Martin-era albums (and the Gillan and Hughes albums, for that matter), are very brooding and melodic... but somehow, they seem to lack that final kick that makes an album like Master of Reality or Heaven and Hell so special. It's great stuff to lie back and listen to, nevertheless. We are, after all, talking about Black Sabbath, so I am complaining on a very high level. I also agree that on the first couple of albums, Martin displayed a wide range, but you know that range and technique isn't everything to me. In my ears, he lacks the charismatic edge that makes a singer like Ozzy, Dio, or yes, even Kiske, instantly recognisable, and he also lacks the expression and purity in his voice that make me love Dio or Bruce so much. And, I just don't like the sound of his voice as much as others (and this is where my absolute hate for Kiske comes in). I like it, I maybe even like it a lot, but still not as much as that of my favourite singers.
Hughes may not be the best singer, but he sang on an incredibly well-composed and atmospheric album, and I think that from that point of view, Seventh Star should be rated at least as high as the Martin albums. It is, in fact, one of my favourite Sabbath albums. Gillan was way below his abilities, though. If you listen to In Rock and Born Again back to back, it's hard to believe we're talking about the same singer. Gillan is in fact one of my favourite singers, and he has all the qualities I listed above, but on Born Again, it sounds more like he got drunk and forgot all about them (which is even fairly close to what actually happened).