I've tried the following :
24, 28, 35, 50, 85, 135mm
The 24mm is nice and gives more context (ie. environment). Its easier to nail focus as well, since it goes to infinity pretty fast, and shooting at f5.6 1.5-2m, it usually needs no focusing. It does require you to be about 1-3m from your subject, so it may not be for everyone.
28mm functions about the same as the 24. Slightly more narrow but not losing context. More stand off distance from subjects as well.
35mm is nice, for the longer 3-5m stand-off distance. It still takes in enough of the environment and is not too hard to nail focus.
50mm is much tighter and the background can start to be less significant due to the more shallow DOF. Nice picture taking distance though. If you are looking for subject isolation w/o being too difficult to get focus, while still being able to take in the environment for some shots, then its a good choice.
85 and 135 tend to fall into the same category. Nice subject isolation, nice distance (I actually prefer nearer than this), but much harder to get focus. Harder to get aperture/shutter speed/DOF/sharpness into a good balance.
85mm
135mm
Don't think there will be anything wrong with 40mm. Just a matter of usage and visual preference.