Black Knight or Legendary Ike

Considering your team can heal up, I would say Ike. You could hit a unit in your enemy team, kill/leave them weak, and Ike can tank and heal himself back up to top via his Radiatn Aether's sustain.

Well the main thing really is that your BK has Steady STANCE, not Breath, which severally limits his kill pressure and due to his low Res will make him rather fragile in most cases.

Normally I'd say get an Armor March on him and then run him next to Surtr as a wall, especially if you can get DC on Surtr as well. But seeing as how you've got just the base kit on everyone, minus the QR2s, then Ike is far superior for you atm.

Warding Breath, though not as great on him as Steady Breath, is what makes him so deadly as an enemy phase unit, allowing for that 4 charge Aether to trigger often enough to not only kill but sustain him. If you want to run BK instead the only way I see him being a reasonable choice is not only getting Armor March to make having two armored units not a detriment mobility-wise, but getting Steady Breath for him so he can proc Black Luna often enough. That should put him ON PAR with L.Ike, but what would make him better is if you can also at some point get Bold Fighter instead of QR in his B slot and then get the QR seal (idk if it's still available to make :P).

However, until you get those skills on BK, your L.Ike is just better not only with your team, but all around as well. BKs speed helps him avoid doubles, which means he has better tanking POTENTIAL then L.Ike, but without those other skills to bolster him he's only average as a unit.