If you have a big budget and want the best monitor for gaming, you should get a monitor with a 27-inch screen, IPS panel, 2,560 x 1,440 resolution, under 15ms input lag, 144Hz refresh rate, and FreeSync
The problem with IPS is it reduces the FreeSync Range of the monitor. When I was looking at my options the only model which had all of the above was the Asus MG279Q, unfortunately upper FreeSync support on that monitor is limited to 90Hz despite it being a 144Hz panel. I was put off by this as I really wanted 144Hz and FreeSync which is why I went for the BenQ XL2730Z instead. Yes, it may be a TN but like LinusTechTips says, “it's not your Grand Daddy's TN.”