Both are important

Both! Not fussed about "high end graphics".. but it does need good visualisation. A good example is super meat boy, anything can run that, but if the visualisation and gameplay mechanics didn't work together the game would have sucked.

Gameplay all the time. Still needs to look decent though. That's why CS is still the greatest.

Pixel optic and good gameplay

both....

Without gameplay game is nothing

Some games are very entertaining although lacking complex graphics , dito Tetris

Gameplay

gameplay! graphics didn't used to be a thing with 8-bits

I still like to dust off the retro games from time to time, and, of course, the graphix are terrible by today's standards, but the games are still fun to play. Conversely, anyone remember Mass Effect 3?...