We all know that GTA 4 had two story lines. The first was:
1) About Niko's past, his friends from the navy and the traitor, Darko Brevic, who killed his friends.
2) Helping Roman out from all the troubles caused by Dimitri Rascalov and paying back to Pegorino about what he did with Niko and his friends.
Both of them were quite good and they interconnected many times like Bernie had trouble with Dimitri, Dimitri making friends with Mr. Bulgarin who is Niko's old enemy, Roman coming into both of the stories as the supporting character, etc. So, what was your favorite storyline? In whole, GTA 4 had a mixture and the best story ever.