It's probably the XFi Fatality for $100, and more than likely 2 years old. Sound is always suggestive, and there's too many aspects to base "Best" on so many opinions.
HT Omega and Asus both have great sound cards concerning Fidelity and slot options. Xfi...are just still there with 6 yo sound cards and 3yo drivers. If you want PCI, I like both the Omega XT and the Asus DS. The DS has the AV200 chipset, decent 107SnR, and an upgradable OpAmp if you want more (it's still limited by the card's ability, so don't think a Burr Brown will make it magical). If you want PCIe, the Asus DX offers all you can feed any desktop speaker system. If you've spent more than $350 on speakers, give it the AV200 and over 115SnR with the D2X.
You could have the best sound card and speakers gifted from the audio gods with the best ear; yet still not tell the difference because you have your ears. So be really true to yourself here. If you just game and listen to MP3's, you want a $75 (and under) card and fit it to the slots you have available. That's most of our members, and the more demanding ask for those added demands they find lacking already...not just "what's on the best list so I can have it too". Harsh, but true. A $280 card with $45 in OpAmps and $560 in modeled amps/speakers doesn't help everyone.
Sound is like picking daisies. They all look the same to the ones just walking by.