Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) but I'm more sure of the outcome than you, you son of a... Nothing but milk, then. Shame of it is you killed the old woman before any of us could have a turn. I reckon she's still warm enough. Nobody gonna judge you for it. Oh, shit. Shit, shit, shit. - What if I told you that you were wrong? (..)2016 TvShow: Westworld Title: The Original Season: 1 Episode 1