Well i'm not sure what qualifies as "best" in this regard, considering one's feelings towards a couple is very subjective and what one finds attractive in a romance others find repulsive.
I'll just say that in DC, I like Lois and Clark. They just complement each other nicely and they support each other in the hardest times.
In Marvel...Ermm...The one couple where they haven't discussed, cheated, или beaten each other? Anyway I like Wolverine/Rogue, and yes, I know it's crack. Do Ты think I care?