Talking about games that push the limits is the kind of nerdy stuff I live for, but I think it's easy to overlook how multi-dimensional "pushing the limits" really is.
In one dimension, you can look at how optimized the CPU program is - whether it's working as efficiently as possible to make the most things happen without causing too much slowdown. Games that push this limit will have things like a bazillion complex moving objects onscreen, or some graphical effect that needs a lot of CPU assistance. GoT on the PCE might be this system's best example of this, but I wouldn't know for sure.
In another dimension, you can look at how fully the game utilizes all the different hardware functions available to it. For example, the Saturn has 5 simultaneous background layers available, two of which can be "mode-7" style, but it's rare to see a game actually use all of them. It's not that it's so hard to "turn on" all five layers in a programming environment, but rather that designing such a background, and having it jive with your game itself and fit nicely in the system's RAM, is difficult. For the PCE, games that use the palettes well, display lots of sprites without flicker, pack the VRAM efficiently with sprites and BG tiles, and use scanline tricks would fit in this category.
One of the more enlightening interviews with Treasure that I've read quoted one of their designers as saying that they've never really done a lot of heavy programming, but rather they've always taken everything that's plainly there in the hardware - no more, no less - and combined it with good design.
Finally, I think another dimension might be when a game uses a system in an unorthodox way. Maybe it could be something seemingly impossible for the hardware, and it could involve undocumented hardware exploits and weird hacks. I'm not really sure what the best examples of this are, though, especially on the PCE. Maybe Art of Fighting and its weird resolution-scaling.
I wonder what PCE game makes the most of all three of these dimensions?
