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.
Ed