I am by no means an expert. But I've been doing a lot of research on that too.
There's a TON of products to consider. Even within a specific company.
My advice? People seem to favor the company Smooth-On. They have soooo many products, it's not even funny. I recommend you email them and give them the specifics of the type of molding and casting you're looking for, what type of equipment you have (some need precise measurements, thus you'll need a digital scale), whether you need purable or brushable, etc. They can recommend one of their products to you.
That said, I think probably platinum based silicone rubber would be good for you. Probably Mold Star (doesn't require scale as it's 1:1 based on volume) would be the easiest and most versatile to use.