What is the best mainboard for a core2quad or core2 extreme

There is no "best" motherboard. Choose one with the memory support and features you want. I use a lowly g31 board cause it's cheap (almost free with a fry's combo) and it would work fine with your cpu. But for overclocking, asus, msi, or gigabyte in a p45 would work fine.

It's not just brand. It's brand and model. All of the major motherboard makers produce highly overclockable motherboards and "vanilla" boards that are poor overclockers.

I happen to prefer Gigabyte. Any of the P45's - UD3L, UD3R, and UD3P - will work nicely.

I agree with those above. I had the P4C with a P4 3.2 and it was a great board, pretty much the board the Asus built their reputation on. There really aren't any boards that are as dominant as that but the thing that made it so good was the Intel i875 chipset (the P4P with the i865 was very good as well). For the socket 775 the best chipsets are the P45 and X48 but there are plenty of good P35 and X38 boards as well. Stay away from Nvidia chipsets for the socket 775, for the most part, they are nothing but problems.