Purely between those three cameras, the K-5 is the best. Best build quality, best image quality, best feature list. The Sony is out because of their non-standard flash mount. Third party accessory makers don't usually bother making their products fit Sony's non-standard mount. Makes it a pain to buy accessories and flashes unless you want to pay far more for Sony's own brand stuff.
Keeping future purchases in mind though I would go with the Nikon. You'll probably want to buy a full frame camera someday (even if you don't realize it now). If you buy the D7000, then someday buy a Nikon full frame, you'll already have some compatible lenses then you won't have to go out and buy all new ones. Some of the lenses may need to be used in DX mode because they were made for your smaller D7000 but at least you'll still be able to use them.
On the other hand Pentax doesn't make full frame cameras, so if you buy the K-5, some years down the road when you want to buy a full frame you will have to buy another brand, which means you won't be able to use the lenses you bought for use with your K-5.
Short term, K-5 is an awesome camera that will whet your appetite for more but leave you hanging when you outgrow it. The D7000 is almost as good while Nikon offers a solid upgrade path for the future.
