Asus Essence STX is the best pretty much, but it's only worth it if you're driving high-end cans.
For gaming, x-fi is the way to go.
While the headphone amp in the Essence STX is good, it's still not ideal if you're truly driving high end cans. And again, if you had the money to buy high end headphones, and a great headphone amp, then you may as well get an external DAC and forget the sound card altogether... unless you want true EAX support, which leads us right back to the Titanium HD. The STX costs more than the Titanium HD, SNR and components are a tad worse, and the only real thing special about it is the headphone amp, which, again, an audiophile will probably pass over in favor of their higher end amp. I just can't see a great spot for the STX currently, ever since the Titanium HD was released. Want EAX for gaming and great quality? Titanium HD. Want the best quality and couldn't care less for EAX? External DAC. Want anything below? Onboard or even the cheaper Xonars.
Don't get me wrong, the STX was the best when it came out, but that was before the Titanium HD was released.