Do you have a budget?
I'm not really a tech geek when it comes to monitors, but apparently people hype up OLED for the best bang for your buck for response times and image quality.
Unfortunately, from the 240hz OLED I've found, it come at a STEEP price (500+$). Had to buy an this one unfortunately. Its works alright, but I could of bought something better if I saved up.
Going off-track; from what I've seen online, people suggesting OLED include Alienware, Asus, Samsung, and LG. Not familiar with any other brands.
Alternatively, zowie has a good selection that isn't OLED. Unfortunately, their 240hz monitors (at least on their website) have been "discontinued?" which also includes its 360hz monitors. But have 280hz and 400hz monitor weirdly? Not familiar if third party sites sell it though. the price is still pretty steep though...