Trooper and Imperial Agent have the best storylines. Smuggler and Consular have been my least favorite.
This...This is a weird post.
For sure, without a doubt, if you want to play for story: Imperial Agent.
People don't scream IA's story without a reason, it is literally fantastic. So much that the writer was hired off Bioware for bigger projects. Don't worry though, he supposedly has written up to chapter 6 for the IA.
I have really heard discouraging thoughts about the Trooper. However, I feel like many expect more war-themes and then are let down when sent on more specific, small strike force teams.