Am... this is a bit of a "forbidden topic" for Bulgarian people...
But to answer your question - Yes, Macedonian IS the most similar language to Bulgarian. However, technically there is no such thing as "Macedonian language" because people from Macedonia were actually Bulgarian people before 1900. Now, they kind of hate Bulgaria...
This is pretty much related to history and I don't want to give long boring explanations.
So, Macedonian is a form [in theory] of Bulgarian and a man from Macedonia would have no difficulty understanding a man from Bulgaria.
Also, Serbian is similar to Bulgarian but not that similar.
Although they share the same alphabet, Russian is NOT that close to Bulgarian.
Thanks for the question. :]