Top 5 things in SEO are:
Content and image optimization
Use good On-page SEO strategy
Try to get Backlinks from high authority sites
Use good Key-words
Competitor analysis
Yes, Title tag is important for a website. It is necessary for both SEO & user experience. It is used to briefly describe the topic & theme of that particular page.
I think having an XML sitemap is great as it will help Google while crawling and indexing the site into the search engine but there is no special use of HTML sitemap.