It depends on your budget and for purpose of usage. If you are having extreme budget and going for 4-way SLI and water-cooling loop i7-6950X is perfect. But if you are not having extreme budget and wanna have processor that can overclock to higher stable clock rates without water-cooling loop go for i7-6850K. i7-6800K is also an option but will give lower stable clock rates at same temp.
1) Best bang for buck is likely the i7 6850K, best is the 6950
2) That program is a joke to run, an i5 6600K and GTX 1060 should run it at 1440p just fine
It depends on your budget and for purpose of usage. If you are having extreme budget and going for 4-way SLI and water-cooling loop i7-6950X is perfect. But if you are not having extreme budget and wanna have processor that can overclock to higher stable clock rates without water-cooling loop go for i7-6850K. i7-6800K is also an option but will give lower stable clock rates at same temp.
The 6900k without a doubt. Being an 8 core ensures that it will never struggle with games(10+ years) and it also doens't lack in the oh-so-important frequency department, seeing as games today still heavily rely on single threaded performance.