Piano

It's the ideal instrument because it covers the widest ground in terms of music concepts, from melody, harmony/chords, and rhythm

Drums is a great 2nd instrument but I wouldn't start on the drums if given the choice as your exposure to music as a whole is focused entirely on rhythm and its interaction with the rest of the band or orchestra - your exposure to melody or harmony is not direct (because you're not playing melodies or chords on drums)

Violin, viola, cello and other "monophonic" instruments like winds and brass (monophonic means you can only play one note at a time - no chords; yes with strings you can play double stops but most of the time it's focused on one note at a time) - the focus is on melody lines which is great, but your exposure to harmony is more limited than piano

Guitar can be a good alternative to piano because you can play one note melodies, chords, etc and as such will get direct exposure to all facets of music

Think of it this way: the ideal instruments to start on are those where you can play an entire arrangement on that one instrument alone - those that can play melody, harmony AND create a sense of rhythm/beat overall at the same time

Also, it becomes a little if not a lot easier to learn 2nd and 3rd instruments if piano is your 1st one