I tried all 4 Javelins during the 2nd weekend, well the 3 others since I had played the Ranger on the first weekend.
My favorite by far is the Storm, his ability to hover for longer, and his elemental abilities just felt more fun to me. It gave me a Warlock Stormcaller vibe from Destiny 2 which is my main, it's very different but felt familiar at the same time.
My second favorite was the Interceptor, again his abilities felt more fun, throwing ninja stars is always cool, and chain dashing while meleing felt good especially against tons of small trash mobs. His ultimate is very similar to the Hunter Arcstrider from Destiny 2 which is my alt. Go all in and kill every while invincible.
The Ranger just felt bland to me, there is nothing wrong with it, but nothing that stood out either. It does it's job with little fanfare which just left me...bored I guess.
The Colossus, I really wanted to like but it just felt broken to me. It's a tank that can't seem to be able to take any more damage than other Javelins but with limited mobility. I think it could be really fun if they make it tankier. I did read though that they admitted there were some issues with it, and it would be fixed for release. I also saw in a few videos that the Colossus really benefits from Colossus specific mods, more than any other classes.
I think there will be a vast overabundance of Storms in the game. Aside from being the most fun(personal opinion) they also look the coolest with that half cape(again personal opinion).