What one that is the best is highly subjective.
Though my personal experience is that arduinos original is the one with the best support. Where you can find alot of documentation about the other ones, you might need to do some legwork updating the code to the latest version of the arduino ide or use an older ide.
Be aware that some Arduinos has integrated ethernet, and others are SHIELDS, you recognize the arduinos with ethernet by the power-connector, and the shields by the distinct name containing "Shield", though shields require an arduino to work. The arduino with integrated ethernet does not.
Also be aware that not all shields fits arduinos boxes if you want to encase it. And of cause, buying arduinos original products, cost a bit more - but you support the arduino foundation, and further development of the products.