There is no simple short answer to that question, it really depends on your budget.
The question is "Is it worth it?" Purchasing a GPU and not being able to utilize its power would be a shame, you have to try and match your monitor with your GPU.
Here's a chart that I made to help you make your decision, but do you own research, look at some benchmarks and find out if you really need to run every game on max graphics settings.
Here are my recommendations, but I am no expert.
1080p@60hz
GeForce GTX 1060 - 6gb
1080p@@120/144hz
GeForce GTX 1070 - 8gb
1440p@60hz
GeForce GTX 1070 - 8gb // GeForce GTX 1080 - 8gb
1440p@120/144hz
GeForce GTX 1080 - 8gb
4k@60hz
GeForce GTX 1080 ti - 11gb
...