i've played titan and warlock extensively (both 28, both rank3 in crucible/vanguard) and i can say there is little difference between the classes, its really your weapons that make up 90% of gameplay. for solo play warlock is slightly better due to self rez in fuckups in nightfall, solar nade spamming being really fun in pvp, and blink being a really cool/fun mechanic for void. for group play titan is really good since the bubble is a must have for raiding, its a super generator for others and 30% dmg boost buff is crazy. You could do hardmode raid I think with 5 titans and a lock or 4 locks and 2 titans, dunno if you could really do it with that many hunters?
titans are pretty lame in PVP, while fistbro mode is fun at first any good player will shotgun you long before you get into melee range and the run mechanics in the game are super annoying. titan shoulder charge requires you to be running for between 2 and 5 seconds to charge, any longer than that and you lose the charge. and of course if you stop running there is a 2 or 3 second internal cooldown before you can run again, so you are flailing around mashing down the most awkward run command and usually die. the melee titan super is probably the second most fun after bladedancer, both warlock supers are kinda lame (wish self rez wasn't tied to it).
titan jump is also the best, its rocket boots so you have really good control where you wanna go for exploring weird areas.
If we are strictly talking PVP.
I would say if equally highly skilled players played any of the classes they can be devastating.
If the same equally skilled players were to play against each other it is very clear that hunter takes less work to become the best.
Hence why Hunters are dominant in pvp on all levels. Their movement, mods, and ability to get in and out of combat without fear of losing makes them by far the most dominant.
2nd place would go to Sunsingers due to the ability to stack 3 shields with radiance, and very good grenades. When they are in radiance it takes 3-4 head shots from a sniper rifle to take them down.
If they are fighting within a mob of people their punch kills almost instantly, and they will stack radiance with its extra shield mod combined with flame skin.
They are the only class that can survive a super to the face. On the flip side their movement is pretty bad, so a person who camps a lot would be perfect for that class.
Titan has very good options as well, being able to switch from defender and striker on the fly. You are not penalized for switching.
A very defensive Titan with a shotgun can be like a lawnmower.
On the PVE end, there really isn't any other class then Sunsinger due to how hard modes work. Everyone else is there to help them out with buffs.
So obviously Titans fit very well into that slot.