The best SS7 mobo I've had would be the LuckyTech P5MVP3 (aka SYE P5MVP3) that's usually seen in some Romanian retro SS7 builds. It's absolutely fast and well documented IMO.
The second best board I've used was Chaintech's 5AGM2, though beware as the PS/2 and USB header isn't standard.
DFI's P5BV3 and K6BV3 are also good ones (1MB cache!) but good luck, those usually had abysmal capacitors (not that any I've listed in my post have any better caps - the SYE board I have was originally filled with Jackons, and Chaintech has some "G-Cap" caps that I don' t really trust but currently am missing replacements for them due to their diameter.) compared to a few other mainboards I've usually seen. About half of the DFIs I've seen had badcaps, and the only one I was able to find again through my life, it was dead due to those caps, unfortunately - I say dead as not even after replacing them, I couldn't get the board to ever work again.
"Enter at your own peril, past the bolted door..."
Main PC: i5 3470, GB B75M-D3H, 16GB RAM, 2x1TB
98SE : P3 650, Soyo SY-6BA+IV, 384MB RAM, 80GB