Susumu, I agree with John. But it depends on what kind of music you are into. Sabbath has taken on a lot of different styles throughout the years. If you are into heavy gloom-and-doom try early Sabbath with Ozzy. There are a ton of albums to pick from. It's just a little hard to listen to for very long.
If you are into a more magestic sound, check out Dio-era Sabbath. Heaven and Hell or Mob Rules are both excellent. I, quite honestly, am quite fond of Live Evil. Great alternate versions of the classics. Dehumanizer is O.K., but not great.
As far as the brief line-up with Ian Gillen, I have never liked him before, and I don't like him in this either. Born Again does not come highly recommended.
Now we step into the more melodic phase of Black Sabbath, starting with Glenn Hughes and the Seventh Star album. An excellent album. Highly recommended if you are a Hughes fan. They made a great team, and had a hit single with "No Stranger To Love". There is also an 8th Star album floating around with demos that didn't make the first album. Not highly recommended.
There are some demos/live material available from the brief Ray Gillen stint. The version I have is called Eternal Gillen. It is some pretty impressive music, but the sound quality is just not there.
Now we move to my favorite period of Black Sabbath's work, the Tony Martin era. This is by far the least popular era of Sabbath's existence, because most people are too stuck on the heavy, plodding sound of early Sabbath. This music is very melodic and majestic, but still has some great complimentary guitar work by Tony Iommi. Eternal Idol is excellent. Headless Cross is pretty good. Tyr is a complete masterpiece. After a short breakup they recorded another brilliant album Cross Purposes. It was followed by a poor album called Forbidden. Tony Martin is a brilliant vocalist and if you are not familiar with him, check it out!
I think that the Glenn Hughes and Tony Martin album would appeal most to a melodic rock fan.