I distinguish between plot and worldbuilding in the following way.
Plot is events and characters directly connected to the main conflict (and maybe large secondary quest lines if we're talking about an "open world" RPG).
Worldbuilding is everything else, from background story to writing style, level architecture and a fitting OST - everything that helps you get in the character.
Examples to get us started (IMO YMMV):
- BG1: great worldbuilding, good plot, meh combat.
- Fallout 2: good world, ok plot, meh combat.
- Arcanum: world-plot varied, meh combat.
- Morrowind: great worldbuilding, good plot, meh combat.
- VTMB: great worldbuilding, good-to-great plot, meh combat.
- Anachronox: meh worldbuilding, good plot, good combat.
Personally I feel like FF7 comes close to be the best all-rounder, though it's not my favorite RPG.
