Loki & Sif
Truth be told? Even now, Loki would probably stab Thor, and Thor would likely punch Loki (either before the stabbing or after). But that was their typical sibling rivalry and nothing venomous like before the thrashing from the Hulk. And despite having a quick temper, Loki would forfeit life itself without a second thought, to save Thor from any real harm. That would always remain unchanged.
Unfortunately, things could change abruptly and without warning. It took only dredging up a hurtful topic from ages ago or some imagined slight for that to happen. It was the realization that could happen, which helped keep things a little more in check. And for all the chaos caused, Loki did not always enjoy being an instrument of change. Devoting one's self to commerce and the arts would have suited the God(dess) of Mischief just fine. It took a clever mind and a silver tongue to arrange and manage such things anyway.
"I do think the both of you would get along famously," Loki replied, smiling and nodding in approval as though it was a pleasing thought that the two should be friends. "I should go tell Thor of this bold plan. Tis a pleasure to speak with you once again, Lady Sif. Good eve."