Finding Different Ranking for a single keyword

Today i searched for a keyword ranking.

In google.com without proxy, rank shown is 13
In googe.com with proxy (from Boston area prvate proxy), rank shown is 4
In seoserp.com , rank shown is 5.

So what is the perfect ranking for that keyword? and why i am getting different ranking from all 3 ways?

there isn't 'perfect' ranking. There are a couple of factors like personalised search, what county you are in and also what area you are in a certain country as the data could be coming from a different data centre

Google results are different for nearly every person, it tracks everyone and tries to give the most relevant results to what they normally click. You could always use something like Google Webmasters to see where you rank on average for your keywords.

Rankings vary a lot on a daily basis. Just build more links to stabilize and claim the #1 spot.