peonpiate: this is a tough subject to say definitively.
all three consoles have various strengths and weaknesses.
the PC-Engine was out first (1987) the SNES came out 3 years later in Japan.
certainly the PC-Engine's on-screen simultaneous color capabilities are far superior to that of the Genesis, released in 1988 in Japan (as the MegaDrive) which must have been very embarrasing for SEGA since overall, the MD-Genesis is more powerful than the PC-Engine (CPU, Sprites, Audio). how Sega could end up with a mere fraction of the on-screen colors compared to the PC-Engine ( 64 vs hundreds) is beyond me.
only with the arrival of the 32X upgrade in 1994, a 32-bit machine, did Sega have the ability to paint more colors on the screen in hardware (without resorting to tricks like HAM), than NEC-Hudson's consoles.
IMO, the MegaDrive / Genesis should've had the same color capability as its big arcade brother, the System16 board; which could do something like 1500~2048 colors on-screen out of 32,768 possible. Also, Sega should've not cut out the scaling chip that was originally supposedly supposed to be in the MD-Genesis. We had to wait until 1991-1992 when the SegaCD added full hardware scaling & rotation.
in the end, there is no clear complete winner in hardware, even when adding in the souped up PC-Engine, the SuperGrafx.
the only hardware that's clearly superior to all of them is the NEO-GEO, minus a couple of effects that NEO-GEO did not have that SNES did, which could be made up in software thanks to NEO-GEO's fast CPU.
I wish NEC-Hudson had released a real 16-bit PC-Engine 2 that competed with the NEO-GEO technically, but at an affordable price.