The honest answer is, whichever order sounds best to you. It's still a given that what works for others may not necessarily be your cup of tea.
There's not a lot of combinations to go through to test out anyhow:
Chorus -> Flanger -> Phaser
Chorus -> Phaser -> Flanger
Flanger -> Chorus -> Phaser
Flanger -> Phaser -> Chorus
Phaser -> Flanger -> Chorus
Phaser -> Chorus -> Flanger
I'd try all of the different combinations (They may or may not sound different depending on your actual pedals that you are using.) and see which one you like the best. It's really the only way to find something that works for you unless you are looking to cop a famous bassist's pedal chain. In which case I'd suggest you searched for the "Pedalboards of the Stars" thread.
