The best storyline IMO is the Sith Inquiz storyline.

So Sith Inquiz could be the most fun to play (who doesnt like electrifying your opponents anyways???)

Jedi Cons is also a very strong Storyline and class

Imp Agent has an amazing storyline but its class is less effective then some others (Its more defined as "here is what your good at" where as other classes have wider ranging abilities)

Trooper is a very enjoyable class to play and its storyline is very militaristic and very well done!

Then we have the Bounty Hunter
Im only on act one (just left Droman Kaas) and the Storyline has been pure unadulterated CARP.

Terrible storyline, boring and mundane as they get
Add to that the Bounty Hunter starts on THE WORST STARTER WORLD THERE IS
I mean, there simply is nothing exciting about Hutta.
Tython has its ruins and Jedi Temple
Ord Mantell is at war and it looks it
Korriban is simply amazing with its old ruins (even if Bioware did totally screw up The Sith Sorceror Naga Sadow legacy by moving his remains from Yavin V to Korriban and having him represent Sith Warriors when he was a Sith Sorceror who utilized Battle Meditation and didnt engage in open combat).

Hutta though is depressing, uninteresting, bland, and void of any appealing characteristics

So with all that going against the Bounty Hunter

Once you get the skill "Death From Above", no other class is as much fun to actually play!

All the classes have better storylines
All the classes (except for Imp Agent) have better starting worlds
And heck, All the classes have better starships

But NONE OF THEM are as fun to just go out and kill stuff play as a Bounty Hunter.