Recommendation for mold putty?

[Hammers]

A question to those of you who have tried this substance. Which brand should I go for?

[snitcythedog]

That depends heavily on what you want the putty to do.  Traditional green stuff is a bit rubbery, and holds shapes well. When you are working on it you can make small changes and have a better chance of not ruining the work that you have already done.  When set it still retains a bit of a rubbery quality.  It can be a bit of work to smooth it into a flat surface.  Water based putties like Milliput or Magic Sculp can be smoothed onto the flat surfaces and can be thinned with water for final smoothing or to change the texture of the putty for different uses.  These do not retain shape well while wet.  If you get a detail that you want and want to smooth the surface around the edges you have to be careful because the wet detail can be easily damaged.  On the plus side.  Water based putties dry rock hard.  When cured they can be drilled, sharpened, sanded and carved.  I hope that gave you some ideas.
Snitchy sends. 

[Hammers]

Sorry snitchy, maybe I was not clear enough? 

I am talking about the kind of specific putties you use to make moulds, not then kind you use for sculpting.

[traveller]

What size of mould are you considering?

[Mitch K]

I've used Gedeo Siligum to cast Fimo clay, green stuff, casting plaster and scratty little bits of white metal. Within the limits of my modelling skills it has worked absolutely fine. This ranges from sword scabbards in white metal to gabions in 28mm scale in plaster, so it appears to be reasonably versatile over a variety of sizes of objects at least. And its available through art and craft shops so it's easy to get hold of and have a play with.

HTH,

M

[snitcythedog]

Sorry snitchy, maybe I was not clear enough? 
I am talking about the kind of specific putties you use to make moulds, not then kind you use for sculpting.

Sorry. 
As Traveller said the scale of the master is a big part of that.  So is detail, undercutting and complexity of the master.  Press molds of fimo or sculpy will work for small to medium size one sided casting.  Instamold is a nice tool for small single and double side molds.  It is cheap and reusable.  There are many manufactures of two-part silicone putty that can be used for medium size castings.  The material is like a two part clay that when mixed will make a quick silicone mold. 20 - 30 min cure time.  For larger molds I use 10-1 liquid silicone.  Manufactures vary by country or even state-to-state in the US.  The cure time is generally 24 hours and it is very flexible and will render high detail.  For low detail, fiberglass molds will work.  All the material needed for those can be found in most DIY or auto repair stores.  Another option for low detail is vacuforming.  A cheap setup works off your personal vacuum cleaner and uses the home oven as a heat source.  I have made molds with each techniques and they all have their pluses and minuses. 
Snitchy sends.

[Sangennaru]

Sorry snitchy, maybe I was not clear enough? 

I am talking about the kind of specific putties you use to make moulds, not then kind you use for sculpting.

i STRONGLY suggest you this site http://www.antichitabelsito.it/
they have various kind of silicone rubbers... but i've bought just now the PUTTY40: high shore, 5 minutes to react, 20 euros per KG... simply awesome.
http://www.antichitabelsito.it/gomma_plasmabile_rapida.htm


for my high quaility moudls, though, i use Prochima GLS50.... well, 45 euros per kg.