There are lot of tools and it keeps on coming, where as the following tools are considered as standard and trustworthy for the newbies and experts already going with them,
* SEMrush - Requires premium for more details - Affordable
* Google adwords Keyword Planner - Minimum Investment is required
* Ahref
* KWfinder.com - Limited usage for free account, but still you can do up to a maximum of 3 KW / Day with free registration. Cheapest one among the above
* Uber suggest - They have changed a lot in the recent. But you can get keyword variations and suggestions - Recommended for understanding the KW variations and formation ideas