I want to buy a new camera and i don't know which one to buy. My favourites are Nikon D7000 and Pentax K-5 Digital SLR. Which one is better?

dpreview has a side by side comparison function.... you can see how these two stack up against each other here.

Best bet is to go into the store and put them in your hands... One might "look" better on paper, but feel better in your hands.

They are both fantastic cameras, So, as McMommy says, try handling them in a shop. You might want to check the price of their respective lenses that you plan on buying which could help you avoid an expensive mistake.
One interesting differentiating point is that the Pentax has "in-house" image stabilization which means you can use old legacy/manual lenses (maybe with an adapter) and still get image stabilization which is pretty awesome if you like messing around those or have a bunch of Takumar or Leica lenses lying around collecting dust.

i use a pentax k5 and it's a good feeling