I want to apply for Ph.D at a good Western university (e.g., in the UK or US) in data science. I have a master's degree (no thesis) and only one publication in a local journal. Will I be competitive for such spots? If not, how can I improve my profile?
The PhD committee in top universities looks for a number of things which includes your publication record, the quality of your references, your work experience, etc. If you wish to do a PhD in a top university you should strive to have a publication in top conferences.
Your publication record in a local journal would be of little help if you wish to be selected for top universities. So you have some work to do before getting acceptance-ready.
Firstly, start by implementing papers from top conferences like ICML and reproduce their results. While you do that understand thoroughly the concepts in those papers. This might require you to revisit your foundations in mathematics. Make sure you maintain your code in Github.
Make a list of top research groups you would like to work with and follow their work. As you follow the step above you will gain more confidence and you would be in a position to comment on their work. Start engaging with them on social networks.
As you start engaging with them you would understand the research direction in which these groups are working. Start reading more of their work and try to identify gaps in their work, which they would also mention in their conclusions and future work sections of their papers.
Try to formulate research ideas which address these gaps and try to write proposals. This would be a bit difficult without guidance, especially if you are from the non-English speaking country. You could use paid service to proofread your proposal.
If it is possible to find a good academician in your country, touch base with them and offer to collaborate with them. Look for someone who already has a decent publication record. By collaborating with them not only could you publish more and gain experience in writing, but also get good reference letter.
There are only a handful of 'good' universities in this world and it is not easy to get into them. But if you work hard you sure will succeed. Best of luck!