The animal whose absence would have the greatest negative effect on the ecosystem is the wolf.
Wolves are apex predators in this scenario, controlling the populations of both deer and foxes. Their absence would lead to an increase in these populations, which could result in overgrazing of vegetation (from the deer), potentially disrupting the entire ecosystem balance. Additionally, the increase in fox population without wolves would lead to a higher predation rate on rabbits, which could further destabilize the ecosystem.