Spotify's playlist recommendations are based on user listening habits, so it's hard to definitively say which are the least popular. However, here are a few types of playlists that might not be as widely listened to:

1. Niche Genre Playlists: These are playlists that cater to very specific genres of music. For example, playlists dedicated to obscure sub-genres of metal, experimental music, or traditional folk music from specific regions might not have as many listeners as more mainstream playlists.

2. Mood-Specific Playlists: While many people enjoy playlists that cater to a specific mood, some of the more specific ones (like "Songs for Sleeping" or "Focus Music") might not be as widely listened to.

3. Non-Music Playlists: Spotify also offers playlists of things like nature sounds, white noise, or spoken word content. While these can be great for certain situations, they're likely not as popular for general listening.

4. Celebrity Playlists: While some celebrity-curated playlists might be popular, others might not be, especially if the celebrity in question isn't known for their musical taste.

5. Local Music Playlists: These playlists feature music from a specific city or region. Unless you're from or interested in that area, these playlists might not appeal to you.

Remember, the popularity of a playlist can also depend on personal taste. What one person might skip over, another might love.