Rain and piano is the pillar most people land on first: a felt piano figure with rain sitting underneath it. It works because the two layers do different jobs. The piano gives the ear a shape to follow, and the rain fills the room so silence never feels empty.
The piano voice is not a sample. It is built from detuned triangle and sine oscillators with a soft hammer transient on each attack, then a lowpass filter that closes as the note decays. That filter movement is what makes it sound felt rather than bright.
The rain layer is filtered pink noise, usually bandpassed around 1.2 kHz, which is roughly where rain on glass sits. Some seeds put the rain further back, some bring it forward. The seeds in this pillar mostly run between 48 and 76 BPM, so the phrase never rushes.
If you are using these for work, pick a seed with a narrow filter and a slow tempo. If you are using them to fall asleep, the lower-tempo seeds with a heavier rain layer hold up better over an eight-hour timer.