This is from my personal experience in SEO.

The weight of the Title tags as far as Google concerns varies in several terms.
Firstly, if you have a 5 word sentence in the title tag each word gets 20% weight excluding stop words like “and” “to” e.t.c.

Then it compares the title with several other terms like your description tag, keyword tag, H1 to H4 tags and basically your content, for relevancy.

So in respect of SEO this could be accounted as the most important tag for onsite SEO. If you mess this up, you did nothing. If you use it correctly you moved to step 2.

In respect of overall percentile, taking in mind that you used the tag correctly, i would give it a 10% weight in accordance with the list of tags you can use for SEO purposes including keywords tag, description, H1 to H4 tags, alt and title tags, and several other elements. In general, it has the most % value than all the rest of the Tags as it is the Beginning of Everything.

Hope this helps