Any era, any pair (or trio).
A few limitations:
[List][*]No Mentor/Sidekicks (i.e. Captain America/Bucky who are narratively bound together)[*]No "Only Cause They're Only Published On The Same Team Book" (like , Beast/Wonder Man because: see above)[*]No Consistently Committed Couples (ala Havok and Polaris)[/list]
So who?