Intel Core 2 Duo E8800 (ES)
Intel Core 2 Duo E6200 (ES)
Intel Core 2 Duo E7700
Intel Pentium II 266 (0,35µ)
Intel Core 2 Duo E7800
LGA 2011-3 platform
LGA 2011 platform
LGA 1356 platform
LGA 1366 platform (Upcoming)
If you want to get the most power for the cheapest price, it is the best choice. Used LGA2011-3 motherboards with X99 chipset from big brand like Asus, Asrock, MSI or Gigabyte are still very expensive on the second hand market like EBay, more than $130-140.
You can get some HP motherboards retired from refurbished workstation for less, like $110-$120. But beware of the compatibility. There were made for specific tested HW, and may not work with some others. And very often they don’t have standard power supply connectors, nor standard form factor, making them impossible to integrate into a standard ATX case...
So, if you want the best for the money, you can forget those options. On Aliexpress, you can get much cheaper or more powerful options.
Three motherboards retained my attention :
X99 Chinese motherboards are the biggest bargain currently for who wants power at the best price.