I use adword keyword planner tool to find best keyword ti site
Always give preference to relevancy, high search and high competition & long tail keyword & you can add place name in keyword.
First study the client website or business details and find out the main services, notedown the all services in a text file and create and then use google planner and find keywords related to your website services search terms which you find out from website. If your clients serves in local area then try to find local keywords include city or state or country at beginning or end of keywords. If you finding keywords by this way you will get best keywords.
Use some third part tools (Free or Premium) like Google Keyword Planner Tool, Traffic Travis, Ubersuggest.org etc. And you must utilize these tools properly to find out some perfect keywords.
A keywords research tool can help you with this task. I prefer to use Google Keyword Planner because it can provide the most exact number of search, while other tools just guess. It also provides tools to narrow down the variety of suggestions that make you finish your task quicker and better