nforce2 was the best for overclocking, as the VIA chipsets didn't allow for fixing the AGP/PCI clocks. If you increased FSB, up they went too.
I had a VIA board for my HTPC, but my 2500+ at stock speeds just was a little too underpowered for HD content, so I bought an ECS N2U400 off of Ebay and oc'ed at stock voltage to 3200+. It could go further, but this board doesnt allow vcore adjustments.
The NF7 is the best as far as I can see from archived reviews I googled. There's usually a few of them on Ebay, at a premium ($80 or so).