Which charger is best for charging iphone 11 pro max to protect my battery health ?

Both will work just fine and neither will be better than the other. Except, the 5W brick will charge the battery slower than the 18W.

Apple wouldn't include a charger which would reduce battery health. The 18W charger only charges marginally faster and slows down after the battery has reached 50% charge.

If that makes you feel better, that's fine.

No. It doesn't. Apple has designed the charging system on iPhone to never overcharge and for the charge to slow the more charge the battery has. You are worrying yourself over nothing here.

The 5w charger will be agonizingly slow and will probably interfere with smart charging. Just use the one that came with your iPhone and be wary of the internet.