Some time ago, I would have said Death Metal, but that's only because back then I thought the only thing that was worth listening to was Opeth.
Personally, I find that Opeth is the only band in the DM region that is really evolving (although this has slowed down a little with Deliverance, I guess we will see what happens on the next LP).
All the bands I have got into in recent times are either Black/Viking Metal (Bathory, Immortal, Satyricon, Blut Aus Bord, [older] Borknagar), or "Post" Black Metal / Post-Black Folk Metal (Agalloch, Sculptured, Vintersorg, Otyg, [newer] Borknagar, etc).
In my opinion, these kinds of bands are the bands who are pushing the creative boundaries of Metal right now. They are incorporating symphonic (and to a limit operatic) influences into their music, as well as acoustic elements (which we know Opeth are doing anyway though), but more namely, they are going into scientific directions and also into directions of nature which are not akin to older types of BM bands.
The only Death Metal styled band I can see apart from Opeth which is pushing creativity is Farmarkon, who, while technically stronger than Opeth, don't have the overall power that Opeth have.
Just my 2 cents, flame away.