Legacy lens mount suggestions

Hi, I'm a new Nex 5N user coming from a Canon 5D and after trying just about every m4/3's camera finally I feel like I've hit pay dirt with Sony's new camera's. My question and I'm sure this has been asked a million times but if I were to go the legacy route which mount would you say gives the best bang for your buck. I absolutely love the peaking feature on the Nex camera's and I'm seriously wanting some awesome fast glass. I'm familiar with canon's fd mount and was wondering if there is anything else out there that would beat them for the money. Y/C mount? K/M MD mount? I think Nikon is too expensive. Thanks much appreciated.