With Cocteau Twins, I don't think theres's a pedal Robin Guthrie hasn't tried. In the early days he used an Electro-Harmonix Clone Theory and well as a Polychorus.
Here's a must read for any Robin Guthrie fan who's interested in effects.
http://www.robinguthrie.com/interviews/ ... player.php
The Clone Theory is probably THE chorus pedal to get if you're going for that early 4AD and 80's post-punk sound. It's also the chorus pedal bassist Peter Hook used with Joy Division.
I own one of the current Clone Theory pedals and I will tell you it sounds really nice along with delay and reverb effects. I actually like it's sound better than the Small Clone, but it's perhaps the noisiest chorus pedal currently on the market. I'm talking lots of white (hissing) noise when the pedal is on. Not much of a problem live, but plan on using a noise suppressor if you want to record with or use a Clone Theory at home/practice.
Now, one of the quietest and best sounding (non-boutique) analogue chorus pedals I've come across is the Jacques Meistersinger. It's a rather simple looking compact sized chorus pedal that sounds incredibly lush and is very affordable. Sounds beautiful with delay and reverb.
On the flanger side, there's the Deluxe Electric Mistress, but the downside is the slight volume drop. Also, some find it a bit metallic sounding. There's also the MXR M117R Flanger which does the 80's flange thing pretty good. You can also pick up a used Boss BF-2 if you want to do the Robert Smith/Cure-style flanging. The Ibanez FL9 Reissue is also pretty good if you want some warm analog swirly flange sounds without going over the top.