I know pay per click marketing is the way of increase the profit for web site but On the internet the program are decided the relative link of search engine please tell me which is the best keyword and why only link building is necessary?
The best keyword is the one with high traffic and low competition Depending on the competition will tell you if link building is necessary. If it's a long tail keyword or a local keyword, then you might be able to get away with just doing on-page SEO and call it good. Usually though you'll need some high quality backlinks.
after the penguin update , if you get good backlinks it is good , but if your get blogger backlinks , you will be susupended within 2 -3 days.