If you have to perform along to a song you've heard only once, which instrumental part (vocals included) do you think you could remember the best and confidently play?
every song is different. in many cases the drums have the least to worry about because they don't have to really learn the chord changes. however, there are many songs where drummer crucially needs advanced knowledge of the road map, breaks, hits, kicks, etc. although this is reasonably easy to memorize by counting. chord changes can be very easy to memorize if you have a large familiarity with common music, and the song is relatively normal. the vocals would usually be by far the hardest to immediately get, due to lyrics