The 980 is old but still a beast, the K6000 underperforms in some cases unless you are purely graphically rendering. I would honestly just go for the 1060 if you want new or the 980 if you want power. If you were willing to buy the k6000, then I'm sure you could buy a 1080 which would outperform all 3 cards.
A 1080 will outperform all three cards in different cases, so if you can, get a 1080. But the others are right. The 980ti is the best of the three cards you have listed. (I updated this as I was reviewing the wrong cards. These things happen with 1 hour of sleep)