Initially I was going to choose Alfred Russel Wallace, because he and Charles Darwin's competition influenced Darwin to publish his ideas and be in competition for who discovered evolution. However without the ideas of Thomas Malthus, neither man would have come up the idea of natural selection which is a key component in the idea of evolution.

Thomas Malthus's Essay on the Principle of Population, influenced Darwin to discover natural selection. In the essay Malthus argued that in nature animals produce more offspring when resources are plentiful and there are little to not predators. In other words population size is held in check by the resources available for the species. Darwin expanded this notion, he states that with two facts from Malthus research, more offspring are produced then will survive and with a limited amount of resources competition must ensue, are all he needed to develop his theory on natural selection. He saw through Malthus how natural selection could be explained.
Malthus Link

Three of the points under "How does Evolution work," correlate to Malthus's influence. Starting with resources are limited. Malthus argued that species are held in check by their resources. In order to produce offspring there must be plentiful of resources, however when famine and other events that devastate the resources occurs the population suffers. Thus there must be a limit on resources, their is not an endless supply. The second is that organisms with better access to resources will be more successful in their reproductive efforts.This is directly stated in Malthus's Essay on the Principle of Population. It was his idea initially that when there is more resources there will be more offspring because the conditions are right and when there is little resources less offspring will be produced. Which leads us to believe that those species with more access to resources would have more offspring then those with out the resources. Then lastly Malthus influenced the point of who gets access to these limited resources? After reading Malthus's ideas Darwin expanded his points based on two facts if there are more offspring produced then will make it to adulthood, and competition for resources must ensue do to limitations, then those offspring with better traits will out compete the others and survive to pass on these traits. 

Without Thomas Malthus ideas on the population and resources, Charles Darwin would have not developed his ideas of natural selection to there fullest extent. It was Malthus essay that inspired both Darwin and Wallace to develop ideas on natural selection and it allowed Darwin to explain not only that natural selection occurred, but how it can be explained. With the influence from Malthus, Darwin could explain that those species with desirable traits would reproduce and survive to pass on those traits. However those species with undesirable traits would not live to pass on their genetic material due to there inability to get enough resources to reproduce.

Darwin's time-frame in publishing his work was also highly influence by the church and the "status quo" of the time. Darwin's family was highly regarded in society in Britain and his wife was also highly religious and so he was reluctant to publish his works in fear of not only dangerous repercussions from the church but also about how his ideas would change how society saw his family. His beliefs would reshape how everyone believed life came into being and so Darwin was hesitant to state his ideas openly. It wasn't until Alfred Russel Wallace sent Darwin his published papers that Darwin knew he needed to get his ideas out there. He knew that if he didn't publish his thoughts then Wallace would get credit for discovering evolution. Thus, the church and social structure delayed his publications, but never his research, which then lead to his eventual publication of his book, Origins of Species.