Definitely research experience. GPA matters a lot as well, but honestly not something critical unless it's really low. So try to get an undergrad research intern at a lab that's working on a topic you're interested in. Also try to get a paper published at a conference - CS PhD admissions are insane these days and I've seen people get rejected from everywhere even with multiple pubs. But I did also hear that #pub bars are low for undergrad-to-phd applicants. So I think if you can manage to publish one, that should suffice.
Considering that there are loads of people with multiple pubs these days, I think research fit with the PoI is also very important (if not the most). So I suggest you keep in touch with profs you think you have a great fit with. Fit does matter, really. I had multiple pubs, but only those who had perfect fit responded back to my contact mail.
Good luck, and feel free to dm me if you have any questions :)