"ii osananajimi o motsuta"

Translation:

"That's a good friend you've got there."

Firstly, this is a panel to prove Midoriya and Bakugou are friends. But it goes even further than that!

Words that could've been used include:
• 友達 (tomodachi - simply put, "friend") or
• 仲間 (nakama - roughly, "comrade/colleague")

but Horikoshi chooses the word "osananajimi". What's the significance?

A very interesting word choice! That's not to say Horikoshi is hinting at them becoming canon obviously, let's not get ahead of ourselves. But the word obviously implies a strong bond, unmatched and unable to be replicated.

If any native Japanese speakers are reading, feel free to add on some subtle nuances to Hori's word choice regarding bkdk, you surely can pinpoint them better than I can!