I think the NEWDESERTROCKS series are the best here. They look the most like a "final" scene. The others are interesting, often good, but look more like texture/material/displacement tests.
I also want to mention that when you render with objects, memory use will go up. So that may be why things started crashing, if you're used to doing *5 renders at once* (which, by the way, is very unusual; I'm not aware of anyone else who does that). If, for example, your renders each typically used 2-3GB of memory, that's 10-15GB. Adding objects will increase that by a notable amount, perhaps another 1GB, which could easily overwhelm your memory. So you may be better off doing "only" 3 renders at a time, for example.
- Oshyan