This is a very tricky one, as I would be more inclined to say gameplay is more important - because it is. However, if a game has poor graphics then I am less inclined to play it. Whilst TS1 and TS2 had good quality graphics at the time they were released and had, without a doubt, amazing gameplay and true depth... if I try to return to play it now, I uninstall after an hour because the graphics are too outdated for me to play with. If the next Sims game was to be released with mediocre graphics for the standard of 2020+, I would be pretty disappointed.
So in theory, gameplay over graphics, but I have voted for graphics.