Off site or on site?

I think both of them are important...

if you want your site to be listed to one of those successful websites then I think both are important.

Are you referring to the techniques and strategies used? The best and the easiest way would be link exchange..

Even if you have a million links you will not rank as well as an ONPAGE SEO optimized page..if your pages are untitled/and or the same for thousands of pages you will be losing out on the power of all those links. You need to have your onpage setup and then offpage. That is not an opinion but personal experience.

both insite or onsite are complementary , without insite work , the outsite linking will be way less effective

Article distribution
Directory Submission
Social Bookmarking

These are comes under off page SEO this is important for page rank and to get good back links.

I don't think one can really tell which eye is important for a body. Similarly, both on site and off-site SEO is a must for higher SERPs.

On-site and off-site are equally important because they contribute to getting your site indexed and crawled. Maintaining both are very effective SEO practices.

both are very useful because first you want to make seo friendly website to make this sites need on page seo and 2nd is you want to promotion your website and you need off page seo for promote your website in search engines.
so both are very useful.

well, to be honest, according to my own experiences--
Off site SEO is just as, or more, important than than the on site SEO. This is getting your name out there in front of the masses so that you can generate traffic to your site while giving something for the search engines to see and rank your page. This is so when someone searches for your keyword you are as close to the top of the first page as possible. This page rank, or page position is all important to being seen more often than not.