It all depends on the amount of sweep you want with your sound.
The MXR Phase 90 is good, but has a limited sweep (IMO), where the EHX Small Stone has quite a wide sweep pattern.
For my money, it would be a Small Stone every time. Especially the "big box" Small Stone, not the Nano, because they look ace. Unfortunately, I've just sold my last "big box" Small Stone, so I'm feeling a tad nostalgic about them.
The Small Clone (again, the "big box" version) is a superb Chorus too. Just sold one of those as well.
The Red Witch Phaser and the MoogerFooger Phaser are both supposed to be great too, but I've never tried either.