I'm Italian and I've been studying Esperanto for four days. I think Esperanto has a mix of words from a lot of languages. Anyway, what is the language that is most similar to Esperanto and who is most advantaged when learning Esperanto?
For the vocabulary it definitely helps to know a Romance or Germanic language, as that is the source of most of them. Anyone that knows a little of a lot of languages (from Greek "kaj" to Russian "krom", Swedish "ju... desto") has an advantage. The correlatives come from Slavic languages, while the accusative is present in a number of Indo-European languages. Here a little more on language similarity from the inimitable Claude Piron. As for my personal experience, my German, French and English knowledge have all given me a leg up in learning la lingvo internacia. After learning Esperanto, I have a hard time getting back to Spanish, so that might be the language it is the most similar to in my head.
Definitively people speaking Romance languages like Italian, Spanish and French. Esperanto shares a lot of vocabulary with them, and in a language where grammar is so easy, vocabulary matter a lot.
I would also say any Western has an advantage: Esperanto is written with the Latin alphabet, shared by many of the western languages, and doesn't make use of concepts like tonality, which many Asian languages use.