Titan: Useful in close quarters especially towards the larger levels. I think that it is also much better at approaching firefights also at later stages. Mobility is one problem I have with the Titan class, although I think it may be something with my playing style and clumsy control.
Basically the titan is balls to the wall jumping into the firefight but you can somewhat make it a lighter class.
Hunters are agile and give what I think is the best super which is the Golden Gun. As a Titanplayer myself, I think it's overpowered, but that could just be my opinion and not fact.
If I can recall, the Hunter has Blade dance which is also very useful, but you unlock thatlater when you're very high in level.
The Hunter is good for agility, and is definitely useful when fighting from a distance. Hunters will get stomped hard by a Titan if they're not prepared for it and dont have their super.
I dont know much about the warlocks. I say go with whatever you think looks most comfortable but don't forget that you can choose another Guardian if you dont like the one you have.