In reality I completed my graduation in 2008. Which line is correct?
I have completed graduation in 2008
I had completed graduation in 2008
I have completed graduation in 2008 is wrong. You can't use present perfect with specific time expressions, and "in 2008" is specific.
As @FumbleFingers says, graduated is a much better term here. The above still applies.
Furthermore, note that the past perfect, versus simple perfect, is optional here for a couple of reasons (read this.), specifically
Unlike with the Present Perfect, it is possible to use specific time words or phrases with the Past Perfect. Although this is possible, it is usually not necessary.
So I graduated in 2008 is the simplest way to say it, but I had graduated in 2008 is not wrong.