I would have to say the Tyan Trinity 100AT (S1590S) for the AT form factor boards. It has 1 AGP, 4 ISA, 4 PCI, 3 DIMM, 2 SIMM, ATA-33, USB, and all the other goodies. It is the most flexible board I have ever had. It is very stable, only down side is it isn't very overclockable (but Tyan boards are known for stability, not overclocking). It also has an ATX power connector.
I currently have 2 computers running with this board. One a K6-3 450 and one a K6-2 350 @ 400. I have also built 2 other computers for other people with this board and so far, no complaints whatsoever. I also know of another person with this board. No complaints!
Just my 2 pennies + a few paperclips.
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