Can be anything, but I always start with either my chord progression or melody.
But usually chord progression.
Even if I don't even end up using the progression in my track, i like to use it as my foundation of the beat. I get my bassline based on my progression, even if I don't use the roots and melodies come to my mind a lot quicker when I already have a progression. I make the drums fits and change them up until I get my vision, sometimes I get unexpected grooves too.