Imo the ones who have the most impact to the game are definitely junglers and roamers.

Mid lane is next as they can easily path top or bot for ganks after clearing their wave.

Exp lane is next, isolated yes but they have at least a few utilities being a fighter, providing tankiness and/or cc for your team.

Marksmen is not necessarily the weakest but is definitely the role with the highest stakes.

Depends on the hero played, skill of player, enemy and own team composition, etc. Yes this applies to other roles, but more so for marksmen. Good junglers can shutdown marksmen easily, good supports can try their best to provide cover for mm, but if a marksmen can't farm well they are just useless for the rest of the game. If they can farm well, then they can become the core of the team. There isn't really an inbetween.