Pick the topic where the search results show beatable competition, where there is real but not hyper-competitive demand, where the subject is relevant to your domain, and where you can deliver genuine information gain. The selection runs on winnable SERP plus relevant demand, not on whatever has the biggest search volume. A weak domain wins by choosing fights it can plausibly take, so the discipline is to pick by beatability rather than by size.
Start with the search results themselves, because the SERP is the clearest read on whether a topic is takeable. Look for results that are thin, off-intent, outdated, or coming from non-authoritative sites, since those are gaps a better page can fill. If page one is wall-to-wall established brands and deeply resourced pages perfectly matched to intent, a weak domain has little realistic path in, no matter how appealing the topic. Beatable competition is the first filter, and it screens out most of the topics that look attractive on volume alone.
Demand and relevance come next, working together. The topic needs real searcher interest, but the most contested high-volume terms are exactly where a weak domain has the least chance, so favor demand that is genuine and meaningful without being hyper-competitive. The subject also has to fit your domain’s existing focus, because relevance is what lets Google trust the page and what lets it reinforce your topical authority instead of scattering it. Finally, confirm you can actually add something, a clearer answer, more useful detail, a better format, because without information gain even a winnable SERP gives you no reason to outrank what is already there.
The practical move is to assess SERP beatability and relevant demand before you commit to any topic. Search the term, read who ranks and how well they serve the intent, judge whether you can do genuinely better, and check that the subject fits your site and carries real interest. If a topic clears all four, it is a candidate worth writing. If it fails the SERP test, set it aside no matter how big it looks, and find the winnable one nearby.