Paid services are always good. SEM Rush, Majestic, Ahrefs, Google keyword planner, Keysearch, KWfinder, and so on.
There are a bunch of them really, but I will mention the ones that I use mostly, and these are Ahrefs, Surfer SEO, and sometimes Ubersuggest. These are all paid tools, but you have free alternatives like Google Keywords Planner or Chrome's extension Keywords Everywhere that are also doing some pretty good job.
A list of free keyword research tools: Keyword Surfer. AnswerThePublic. Keyword Sheeter. Keyworddit. QuestionDB.
best keyword research tools: SEMrush. KWFinder. Ahrefs Keyword Explorer. Google Keyword Planner. GrowthBar.