It all depends on the features you need:
-- Intel or AMD CPU?
-- PCI slot yes/no?
-- Total expansion slots (PCI/PCI-e)?
-- 2 or 4 RAM slots?
-- 16, 32 or 64 GB maximum RAM?
-- Desired RAM speed?
-- Number of SATA ports, & type (I, II or III)?
-- IDE support yes/no?
-- USB 3.0 yes/no, & #?
-- Extra USB 2.0/3.0 internal headers?
-- Onboard graphics yes/no?