Yes, the resolution of the largest image makes sense.

However, really, these days, this is barely an important characteristic — all cameras on the market are at the "yep, that's a lot" level or beyond. It's true that more megapixels can be good, but in order to take advantage of them, one needs a very good lens, a lot of light, no camera motion, and so on — for most use, the difference between 16MP and 20MP is negligible. (As a rule of thumb, I'd suggest that 50% larger in each dimension is a significant difference — so, more than double the megapixel count.)

You could go with the "perceptual megapixels" score introduced by DxOMark — this tries to give a meaningful number for the practically perceived detail for a camera and lens complication. But to me, this is a lot of extra complication, and overall I'd just deemphasize the number overall — maybe lumping cameras into 10mpix-ish, 20mpix-ish, 40mpix-ish, and so on.

To be really helpful to "end users" (I'm not sure exactly what that is in a photographic context, but I'll take it to mean beginner/intermediate photographers), I might even suggest grouping the cameras into high, medium, and low pixel count with a * noting that the designation is relative to other cameras of the same approximate age and with the same sensor size. For today's mid-range cameras, that might be

• Low: < 12mpix
• Medium: 12-20mpix
• High: 20mpix+

but you might want to use different scales for camera phones (with 8-12mpix being medium) or large-sensor full-frame cameras (with medium at 24mpix, say). In any case, the key is to use a generious range — don't break it down more than is meaningful.