Since we're talking sidescrollers, i voted gameplay and sprites.
CGs i focus on more in rpgs as they are less hectic, and i've played enough games from lurking around here over the years to know good gameplay can be hampered by weird or bad sprites, and great sprites can be hampered by awful gameplay. ....especially the latter, which is kindof a plague in h-games at times.
Plenty of games with good sprite sex or CGs but have gameplay where the movement is slippery and/or the hitboxes are wonky, and they end up being the ones you give up playing, download the imageset, fap once, and archive it away never to be seen again.
Other way around, end up being played but i keep thinking "man these sprites or blobby and awful, i wish the better looking games had this gameplay though. is that thing actually fucking her or is it doing some other more niche fetish like forced eating or something even weirder? i can't tell! there are 17 colors here, none of them are skintones, and all of them are undulating! gah!"
CGs can be done later. specifically speaking on them though, i find games less enticing when the cgs get made too early, and then the scenes or situations get adjusted later and then they don't match. (or no one cared to match them up in the first place...)