Lorne Greene vs. Edward James Olmos
(I'm going to assume this is a comparison between their performances as Adama, rather than a Chief Rorchek vs. Lt. Castillo debate.)
I think a lot of this one comes down to how close you were to the original source material. For those of us (yeah, I'm that old) who can remember the original BSG airing on TV, Lorne Greene was perfect in his role: that deep-timbred, serious voice of authority, carrying with it all of the associated gravitas of Ben Cartwright (that's a "Bonanza" reference, kids...look it up). I can remember watching those episodes and being hard-pressed to imagine anyone else who could carry off being Adama without making it seem unconvincing or downright campy.
With that said, when the re-imagined BSG came out, I was immediately sold on Olmos. He was serious to the point of being curmudgeonly; gravelly-voiced, weathered-looking, a man who life had put through the physical and emotional wringer, and was a walking piece of surly sharp-eyed gristle. Where Lorne Greene was the right Adama for a disco-sparkly, Tab-drinking, Carter-led America, Olmos was the right Adama for a disillusioned, dysfunctional, off-balance post-9/11 America. He could play a role that had become tougher, meaner, and existed in a far more morally-ambiguous universe than that of its predecessor.
So: who did it better? Both actors were best-suited to their respective times, but I think that Olmos had the more challenging Adama to portray, so I'd say he'd be my pick for the better of the two actors.