All three classes are equally good but if you are brand new to destiny 2 it is recommended that you choose the Titan as his stong defensive and offensive capabilities make him a powerhouse in PvE and PvP activities. Ultimately you should base your selection on your playstyle and pick the class that best suits that. Whatever class you decide to go with you are not obligated to see it through to the end if you realise your selection is not working for you as you can always make a new class.
The Titan Class will suit players who like to play a bit more defensively but still want power behind their hits. Their weakness though is lack of mobility so be prepared to hold your ground if necessary, especially in PvP focused battles. The Hunter Class will be for players who favour being quick and mobile and pride themselves on being accurate with their shots. And finally the Warlock Class which will suit players looking for a class that is good at both supporting and wiping out waves of foes.