Physically, cast are weakest, followed by milled (machined-from-billet) lowers, with forged lowers being strongest.  Typically, though, machined lowers are stronger, because the walls are made thicker (these are also MUCH heavier).  So you aren't exactly comparing apples-to-apples in most cases.  Milled lowers are usually only used for match rifles, where their extra weight is an asset instead of a liability.

As DevL stated, the difference between forged lowers is mostly about where and how they differ from Colt SP1-style machining.  In other words, what kind of sear block or unmachined area is there in the lower, and in the case of a Colt lower, what size fire control and/or pivot pins are used (Colt is the only manufacturer to use non-standard sizes in civilian rifles).  Virtually all ARs today except Hesse/Vulcan use proper hard-coat anodizing as a finsih, though older rifles from some vendors did not.

-Troy

All receivers are machined.  The best (in general) are machined from aluminum forgings.  Forging just gets the parts close to the right size and shape.  Usually the exterior won't need any work, but the inside sure will.  Forgings aren't magic, however, and you can get bad ones.  Not likely.

-- CHuck