Gunnar is undoubtedly great for everyone, but if we're talking about a hero for your main Castle, I would still prefer Kiryu for Rampart.
Added 5 minutes later
Here's another interesting question, in case anyone doesn't know: how is the information about the strongest hero updated in the tavern during the opponent's turn? I recently learned that 0) only the starting army is counted, 1) it is updated with each hero purchased, and 2) heroes with units of 1 have their potentially available troops taken into account. I started checking – it doesn't update immediately after purchase or appearance, even hiding in a garrison is not reflected. I came to the conclusion that the new hero needs to take a step out of the Castle for the update.