What is the best SEO tool for launching a new website?

Use:-

Google webmasters

Google Analytics

Google Keyword Planner

Yoast SEO

Ahref

Google Trends

We could use all these.

Create page titles. Keyword-based titles help establish page theme and direction for your keywords.Place strategic search phrases on pages. ...Develop new sitemaps for Google and Bing. ...Submit website to directories keyword planner its free you can use screaming frog which a tool its usefull  buzzmo and other sites are paid services 

There are  different  types  of   off page seo  tools are available . for example if you want to explore your competitor Backlinks so I will suggest Moz explorer which is free & paid from free version you  analyize some of links of your  competitors . to explore  more you will need to buy paid  version  of that .  same like for  other  purpose there many tools available to explorer other contents , guest blogging and etc