There are a lot of threads covering this subject, so a search will pay off.
Chorus is one of the most popular effect discussed in this forum.
In short, the Boss CEB-3 and CE-5seem to be the most popular (if only looking at Boss pedals), the first for its subtlety and the second for its wider possibilities. But there are other options. MXR pedals are at least as well built as Boss pedals (IMO). The Analog chorus is a popular pedal, but MXR recently anounced the arrival of its new bass chorus (New MXR pedals
Personaly, I really like the MXR Stereo chorus: it has a sparkling quality, is very tweakable and has a bass filter switch which help preserving some decent bottom. I've also seen the 1-knob Micro chorus on a few boards. Also: Danelectro, Analogman, Digitech are also brands you'll see around here.