Let me start by saying that if you are going to make very standard types of molds, if might be less expensive (and less frustrating) to buy silicone molds from some of the vendors who frequent this website. I always liked the molds sold by Fred Wissen (PTownSubbie) luv2turn.com, but there are others.
If you just want to do it yourself, or you have non-standard ideas then I've worked with the following and found them satisfactory:
Smooth-on: Mold Max 25 and OOMOO. OOMOO is good for quick mold prototypes, but not the best for hard working production molds.
US Composites: 70-25 Tin Sil.
Alumilite: High Strength 2
They are all expensive (to me anyway) and don't differ all that much in price.
My current favorite vendor is Smooth-on because they have the widest selection of different rubbers for every purpose you could imagine. If you've never worked with 2 part silicones, don't buy a lot to start. Make sure your first time using it won't be your last before you buy a lot.
I'm not a mold master by any means. I'm just having fun with the stuff, so my opinions should not be taken as being anything close to resembling expert advice. But they are probably worth what you are paying for them.
Cheap = Silicone Caulk. There is a deal in the library that shows to mix it in soapy water and it cures very fast. I have made them as prototype molds and they work ok. Very stiff so make sure your walls are not real thick.
Expensive but last = Silicone Rubber. I use Mold Max 30 and can get 100's of casts out of one mold. Then when it starts sticking just use some Dupont Silicone spray as a mold release. It is much cheaper than actual "mold release" and you will get 3 time more in the can.
Well so far I have tried PVC pipe molds, molds made of UHMW and wood and a rubber mold. It was a 2 part rubber left over from some project or other my wife did with the kids. The PVC pipe is relatively easy to use, but I don't get a great swirl on the color mixes. It also is tough for embedments. The UHMW I could not get it to stop leaking. I tried various things to seal it. The wood I lined with plastic wrap. That worked except it is tough to get square corners with the plastic wrap. The rubber seems to work the best - it doesn't leak and when the blank is cured I can just pop it out.