High organic rankings are achieved by good SEO, not good sitemaps.
The most important thing to remember about XML sitemaps is that they’re only as good as the website they represent.
In a way, an XML sitemap in and of itself isn’t SEO at all – instead it simply reflects the SEO on your website, good or bad. Sure, XML sitemaps need to be formatted optimally. But the most amazing, well-formatted XML sitemap means nothing if your on-site SEO sucks. What XML sitemap protocols one uses (outlined here http://www.sitemaps.org/protocol.html), how often the sitemap is updated, etc. are all important, but nothing compared to good overall SEO.
For instance, even if you give priority to one page vs. another in your XML sitemap, Google is still going to determine on its own which pages it prioritizes based on… you guessed it: SEO (which pages are best optimized, have the highest page authority, etc.).
That said, you always want the search engines to index as much content as possible, provided that it’s well-optimized content. And a good sitemap helps toward that end. But at the end of the day… good, compelling, optimized content is primary. If that’s the case, then Google will find that content one way or another, with or without a good sitemap.
Search engines can easily find pages even if they’re not in your sitemap – when a page is back-linked to from another page / site, for instance. Conversely, listing a page in your XML sitemap, does not guarantee the search engines will choose to index or rank that page. Yes, the search engines want to know about all your content, but they decide which pages get picked up and rank high, based on one thing: good on-site and off-site SEO.