It depends on what you really like... My first main character was a Warden - which I found incredibly fun! And when it came to trying other classes nothing seemed be quite like it. I've always hated Melee classes myself, so Warr and Pally were out. So that left me with 4 other choices. (Before Bard and Engi came)
I tried Summoner, and found it way too slow in combat (this was before the rework, mind you) for my liking. It has a pet and heals which is helpful but just not my cup of tea.
Then I tried Mage. Always loved Mage classes in MMOs, the glass cannon, the top dps class, the class that I have always known. And I was so bored. Of course it's changed now, my Main on Sub Server is a Mage currently.
Next came Scout! It was great until 25ish. The lack of Melee didn't seem to work when it came to mobs getting closer. At the time I played a scout it was still a very hard class to play, with Depleted Resources and not being able to cast anything up close. So it's easier and more versatile now I guess, but the initial impression I got from the class has kind of stuck.
Finally, I tried Psi. I loved it. I absolutely fell in love with the class. The cast times were kind of annoying but being able to have a Mental twin and Manipulate my enemies (even if it was just my mind RPing the Scenario) was great! Also, being one of the few Psis around at the time was very fun - it gave you a sense of Pride and Prestige. I played a Psi as my main until 3.0 where I decided that the situation the class was in wasn't going to improve. The devs then reworked psi a few patches later but by then I had another main.
I don't know if this post has been helpful to you - but in sense, just try out everything! You're bound to find another class you'll like (play into the 25-30 area if you can) if you are willing to spend a few hours with each character learning how to play and which skills to use when.