There's actually a pretty easy answer to this question. But it takes a while to get there.
First, take into account that bayer sensors don't resolve the full resolution of the image you end up with. They resolve about 70% in each axis to nearly 100% mtf, then alias a bit, then mtf drops to zero. But with sharpening (since digital is so low noise) you can boost this up so it seems higher. Remember, perceptual sharpness is most accurately represented by the measure of the area under the mtf curve--NOT where that curve hits extinction--and sharpening can bring areas of that curve over 100%. This means 25 megapixels is really closer to 12 "perfect" megapixels, but in practice seems significantly sharper than that.
Take fuji velvia 50, the sharpest color film; it has 100% mtf up until the equivalent of 4-6 megapixels, I think, (not going to bother to look up numbers, could be more or less), then hits extinction at about 25 megapixels. And my scans back this up.
Depending on how you measure (mtf going under 100; mtf reaching extinction; area under mtf curve), you'd get something like 4, 12, or 25 megapixels from 35mm slides is my guess. I'd put APS-C digital on par with 35mm film but much faster and full frame digital on par with 645 or 6x7 but faster, just personally. I prefer 6x7 chromes to full frame digital, but once printed they look pretty similar to me!
But film also has a LOT more grain. Taking SNR into account, film takes a huge hit...
Fwiw, black and white is even sharper and c41 is generally less sharp but not bad at all!
But film, shot properly, still looks better! Love the colors and highlight roll off.