Final answer:
Britain was the imperial power that financially benefited the most from Argentina's extensive rail network and agricultural exports, such as wheat and beef, due to its significant investments and market control in the region.
Step-by-step explanation:
The imperial power that most significantly financially benefited from Argentina, with its well-developed rail network and abundant supply of agricultural products like wheat and beef, was Britain. During the phase of neo-imperialism, British investors had a profound impact on Latin America's economies, including Argentina. This influence included extending significant loans to the new Latin American governments, leading to a dependency on British finances and the supremacy of British goods in their markets. Argentina's economy was thus intertwined with British economic interests, particularly through the export of beef and grain which were staples in the British diet. This commercial relationship favored the British, who reaped the benefits of Argentina's raw materials while also capturing the market for manufactured goods in the region.
In the 19th and early 20th centuries, Argentina's railroads were primarily built to facilitate the export of these raw materials to ports, thus benefiting British commerce. Despite having a rapidly flourishing agricultural sector at the turn of the 20th century, which made Argentina one of the wealthiest countries at that time, the country still relied heavily on foreign investments and the export economy. The boom in the agricultural export market led to the prosperity of the Porteños in Buenos Aires. The wealth disparity in the region was accentuated by the class divide between the affluent who could afford imported goods and the laborers who could not.