In programming, the symbol " != " means "does not equal." Since that's a legal statement, the program compiles and runs. For most software companies, that equals a success and it gets shipped.
Now, as for my answer... I really have no idea, as most of these are linked. The Pendant, the Mammon Machine, and the Ocean Palace all figure into one of the largest pieces of the story. The Chrono Trigger is important, if not vital. The Masamune is, well, really really cool. FATE is the driving force behind the majority of Cross. The Epoch is a major transport, and the Sapling of Hope kicks off the best scripted sidequest in the game, and let's you save Lucca's mom.
*sigh* I'm gonna go with the Mammon Machine, as it was built to harness Lavos, which is what kicked this whole show off. The Pendant is very important, but it mostly acted as a magical key. The Mammon Machine, however, is pure power.