Most powerful Redwall Hero (out of the nine)

[The Skarzs]

Out of those options, Lord Brocktree.

[Russa Nodrey]

Brocktree or Martin.

[Mister Cheese]

I would think Brocktree if we're talking about physical strength here. However, you can also have strength as in influence or strength in your character, and to that I think I'd say Martin. Because Redwall has a tapestry of him and they would never let it get stolen, just because they revere him so much.