My personal opinion is the best chorus for bass is from Analogman
http://www.buyanalogman.com/Chorus_Pedals_s/42.htm. They have several models but with same circuitry. First off, this is truly a fantastic chorus for use with a bass. Incredibly well-made, a solid build quality and attention to detail. "Analog" Mike suggested I use 18v for increased headroom as I was going to be using it primarily with my bass. Great advice. The sound is fantastic! But, can also be used w/9v or 12v as well. Most times I have it set with Speed at 11am and Depth at 2pm. The controls are quite sensitive so you'll be able to find "your tone." I also had it built with the "Depth Toggle" 3-way toggle option. This is a must-have option for bass. In addition, you can flip the toggled "Deep Setting" off for use with regular guitars. The DEEP option, is a a three position mini toggle switch. The DEEP setting not only makes the chorus thicker at all settings, but moves the frequency spectrum of the chorus down - perfect for bass!. This enhances the low frequencies. Sounds great on bass and excellent on guitar too, if you want to be able to dial-in thicker chorus sounds. The second (middle) position is a THIN setting, which sounds a little like the tc electronics Chorus. It is a lighter sound, with less low end, and also a little less analog white noise. It sounds a little flanger-like as a flanger has a shorter delay time than a chorus. The Third position is the exact same setting as their standard CHORUS, with no extra noise or change in tone from their normal pedals.
The middle position of the toggle switch is always the THIN setting. On their standard chorus, DEEP is down and NORMAL is up. On their Bi-chorus, LEFT is deep and RIGHT is normal.
If you've never dealt with Analogman be advised that their customer service is amazing. "Analog" Mike is readily available. You would be hard-pressed to find a more customer-focused operation. Their rep is well-deserved.