What makes for a "strong" LoR for graduate application?
That is highly dependent on the thoroughness of the committee doing the ranking, and other random facts (i.e., one might hate the guts of the renowned colleague, or have met and been well impressed by the fresh professor). In case of doubt, I'd go for someone who can really talk about you (worked together, took classes with them, ...) than some star who at most will say they've met you and you seem OK.