I'd start with the basics, google keyword planner and alexa.

Keywordtool.io for me because it also show me keywords for youtube ,bing etc .But for seo purpose keyword planner its the best i think

Try those: longtailpro.com, ahrefs.com and keywordrevealer.com.

i used keywordtool and keyword planner

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Spyfu.com is good

For keyword research here is my suggestion Ahrefs or Kwfinder but only use them to get an idea after that do the task manually to get a better understanding.

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Google Keyword Planner - This is free
SEMRUSH - 14 day free trial is available.

Here is list of best keyword research tools:
Google Keyword planner
SEMRUSH
Ahrefs
LongTailPro
KWFinder
SpyFU
WordTracker
SERPStat

keywordtool and keyword planner , these two are working better.:

SEMRUSH ,Ahrefs ,LongTailPro,KWFinder ,SpyFU these are some of the keyword finder

google keyword planner and alexa.