What sitemap tool you would recommend me for site with over 10M pages.

You can only have 50k URLs per xml sitemap file, after that you need to split the sitemap. You can also refer to "Google sitemap generator" for more information
http://code.google.com/p/googlesitemapgenerator/

Site map is very important for seo onpage, From site map get index page pagein search engine

I don't think there would be any tool which would be able to generate sitemap of 10M pages. instead you need to adopt text sitemap which you can easily create on your own. You just need to list all your urls in simple text file and save it to UTF-8 format and upload it just like XML sitemap.

You can only have 50k URLs per xml sitemap file, after that you need to split the sitemap. You can also refer to "Google sitemap generator" for more information