PvE or PvP? PvE I'd say all of them are just as difficult or just as easy. You can do anything in PvE with any class if you put your mind to it. PvP I'd say Defender Titan. With how the meta is right now with the dominance of Blink, Tracking Grenades, and Extended Melee ranges the Titans are at a disadvantage but can still be done if you really put the effort into it.