what's the best casting resin?

I like Smooth-On's line of RTV and urethane resin.    I use OOMOO-25 RTV and SmoothCast resin.   The resin is water-thin so pressure/vacuum casting is not an absolute requirement. 

http://www.smooth-on.com/index.php?cPath=1290

SmoothCast resin hardens with a minumum of surface bubbles - that is something which always plagued me with Alumilite products.

Check the starter packs at Smooth-On.  Good value for the money, complete with instructions, tips & hints.   Much is also available from the website too.  Technical people at the home office to talk you through problems, as well as technical support through their distributor network. 

Micromark is actually one of their distributors  (see the Smooth-On distributor list).  I buy over the counter from my local distributor & it saves me a buck or two on shipping.

To follow up with what Ed said I like the Smooth-on as well.  My friend Hugh at Loose Cannon turned me on to it.  A tip he taught me is to add dye that Smooth-On sells to tint the resin.  It's available in different colors.  I add about 3 drops of black dye to their part B bottle (4 pound bottle) and you get a marvelous shade of primer gray in the finished casting.

Dave