Superman/Clark Kent -
The good boy alongside someone who can save the world. The strongest, fastest, bestest but at home (or in private) just a down home farmboy.
Batman/Bruce Wayne -
The dark knight. That basically says it all. You don't really know about his ideas of good and evil, they could be completely skewed; he could be completely psychotic, cold, distant, and incredibly rude. But in private, he's a brooding man who's just waiting for the right person to come along and unlock the emotional floodgates.
Me personally, I find myself attracted to:
Battousai/Himura Kenshin -
The rough, dangerous, almost animal like, and incredibly instinctual Battousai who can just as easily chop you into chop suey as any person takes to say their name. But Kenshin is the altruistic, kind, gentle rurouni who is playful and humble and can fumble quite a bit.
Hrm, I'll develop on this when I have the time...because there are a LOT more out there.