For me it would have to be the Klingons in TNG and the Ferengi in DS9. I split it up because both races were given such vastly differences representations on each show it seems silly to attempt an overall comparison. I like the honorable, tradition based culture of the Klingons on The Next Generation as well as the episodes involving Qo'noS. I like the Ferengi the most on DS9 mainly because of Quark and Rom but also because of the different aspects of their culture and personalities we get to see. Not all are greedy, ultra-capitalists, Rom, for example, is very family oriented and Nog is trying to get into Starfleet. now that I think of it the Trill are a close second.