That's a duh question, 980ti if you can afford it. There is nothing better than it presently.
Edit:
But, to be honest, I'd get a higher resolution screen and save money on the GPU, also 24" seems like a very small screen?
I'm running 2560x1440p, windows no longer seems to have scaling issues and I can run all my games at max res (except for arma...) with a 970. You could always get a second 970 later on, it'll be cheaper than the 980ti