Hmm... metal?
No, seriously, they've done so many different kinds of metal that it's hard to pin them down to just one or two subgenres.
Their original stuff (early '80s) was very... '80s Japanese metal. Pretty heavy for the time I guess, but it wouldn't be considered heavy by today's standards.
Then people got annoyed because in the mid-80s, they started heading toward what people said was a more "US-commercial", "radio-friendly" metal sound. It did sound different from their previous stuff but to be honest I think they had some great songs during this period too. They also changed vocalists and had an American (I think Mike was American, wasn't he?) singer for a couple albums.
Then in 1992 there was a HUGE overhaul of the band, both in terms of members and musical style. (This was when Taiji briefly joined them.) The 1992 self-titled album was way heavier and grittier than their previous stuff. It was also pretty damn cool.
And Arucard will have to help you with the next "eras" of Loudness since I pretty much missed them so far. :?
But Metal Mad (the latest album, released two months ago) is heavier yet than their 1992 stuff, anyway.