My most important playlists (outside of pre-set ones like Recently Added or Played):
Year: decade, 60's-00's
Recent Favourites (4-5 star songs played in last 2 weeks, limited to 100 by most frequently played)
Favourite (Genre) (3-5 star songs in Rock, Reggae, Jazz, Classical, etc.)
But my most used is my "Main Playlist". This is a dependent playlist compiling 10 other playlists, which excludes genres I don't want popping up randomly (Classical, Instrumental Jazz, etc.) or songs that are too long (over 15 minutes), or albums I like to listen to end-to-end (like Dark Side of the Moon).
Of those that are left, it gives me a mix of roughly 37.5% 5-star, 37.5% 4-star, 17% 3-star and 8% 2-star songs, that haven't been played in the last 3 days/7 days/14 days/28 days, respectively. It also limits the number of songs from artists which I have a ton of to a reasonable number, so every 3rd song isn't by the same 3 or 4 artists.
I love smart playlists.