There's no single best keyword tool. Most pros use 2-3. Here's what each of the major ones actually does well, and where they fall short.
Strengths: best-in-class backlink data, strong keyword database, accurate traffic estimates, excellent content gap feature.
Weaknesses: expensive, somewhat dated UX.
Use for: competitor analysis, content gap, backlink-based difficulty estimation.
Strengths: broad feature set covering PPC and SEO, good SERP feature tracking, solid keyword suggestions.
Weaknesses: reporting can feel bloated, data occasionally disagrees with other tools.
Use for: all-in-one SEO workflows, especially when you're also running PPC.
Strengths: free, uses Google's own data, essential for PPC.
Weaknesses: volume in wide buckets (e.g. "1K-10K/mo"), weighted toward PPC use.
Use for: double-checking volumes from other tools, PPC planning.
Strengths: free, your own actual query data from Google, the only truly accurate source.
Weaknesses: only shows queries your site already gets impressions for.
Use for: optimizing content you already have, finding queries you rank low for but could push up.
Strengths: free, shows search interest over time, regional comparison.
Weaknesses: no absolute numbers (just relative).
Use for: seasonality, emerging trends, comparing the popularity of two queries.
Strengths: clean UX, good for small-medium businesses, intuitive difficulty scores.
Weaknesses: smaller database than Ahrefs/SEMrush.
Use for: getting started, mid-market users.
Strengths: cheap, good for basic research, from Neil Patel.
Weaknesses: data quality uneven, limited enterprise features.
Use for: solo founders, bootstrapped projects.
Specialty tools for keyword clustering specifically. Worth adding to the stack if your list is >500 keywords.
Content optimization tools, not just keyword research. Take a keyword, tell you what terms to cover, what word count, what structure. Useful in the content-brief stage, not raw keyword research.