I played the Titan the entire beta, I never felt the experience with the Hunter and Warlock, But after my experience with the Titan, I can say that this class is like a Tank pretty much with the right skill and subclass skills and equipment. I like the Hunter because It's Super Charge is a 1-hit death gun, (Idk the name) But It's like gold at a distance cause I get hit with it a bunch. Damn that gun is crazy. Anyways I think the Titan is a great class. That's just my saying.

It all depends on how you play, As weapons aren't class locked (In the beta anyway) - In any case - I prefer the warlock (My opinion) <- Why? Because I play better as a caster/spell caster in most mmorpgs. There's no "Best" class, unless it's your preference tbh.

When the full game is released then we will know what class is truly the greatest.
Most likely, the class that is great for your fellow guardian will not be great for you.
The classes are built around your playing style so you need to find a class that suits you.
Not others...