I have done both Colonix and Michael's Ultimate Detox and Cleanse and I've been very satisfied with both of them. Colonix will flush you out but you'll also want to have the convenience of being around a toilet at all times just in case the urge comes up, and when it does, its intense! The Michael's Ultimate Detox and Cleanse is more gentle and if you are nervous in any way about an intense one or you just don't have about three months to do a cleanse then the Michael's is the way you want to go. It's only for two weeks and its gentle and affordable. You can get it online but I'd recommend you go to Whole Foods if there is one nearby.
Again those are the only ones I've done so they are the only ones I can vouch for. The only thing I can definitely say about all cleanses are that they need to be completely natural, no weird chemicals in them or they won't actually be cleansing your system, only adding more chemicals and toxins or at least replacing them.
If you do have the time and money though, and are willing to go the distance with a longer period cleanse, I would DEFINITELY go for Colonix. Just look at some of the pictures of the stuff that comes out of people when they do the cleanse! WOW! (just keep in mind the stuff came out of a certain area, so don't expect it to look like flowers and unicorns)
Oh and another random tidbit of information for women: During and after doing both cleanses, my periods were a bit less heavy and I barely had cramps anymore and I mean I had crippling ones before the cleanses.