Titan - Beefy, takes more damage to kill, better defensive, plays like more of a brawler if you'd like
Hunter - potentially super high damage, very agile, requires constant movement and maneuvering to not get destroyed in PvE
Warlock - self sustaining, little slower feeling than the other classes but makes up for it with damage and utility, really fun supers
In the end, I'd recommend you play one of each to get the most out of this game and avoid getting burned out quickly. But as a first choice, it's up to you. I'd say Titan is more forgiving, but I went Hunter first as it fits my playstyle and it worked out great. So, just see what fits your playstyle