In terms of PvM, Roland is probably the best because the primary skill he gets allows him to replenish ammo and health while taking a lot of damage, which can be extremely helpful when being attacked by mobs of enemies. However, in terms of PvP, Mordecai is BY FAR the best because you can literally make his Bloodwing skill 1 hit kill any other player even if they have full health and shield. All you have to do when playing as Mordecai is look at the person and they die from Bloodwing, so there's really no escape unless somebody is playing as Lilith and manage to phasewalk at the right moment (but even so, the Bloodwing can also daze you so the player will know where you are even though you are invisible). If that's not overpowered enough, Mordecai can also make it so his guns ignore shields and at higher levels, a good gun can also 1-2 hit kill anyone. Despite this, I still like Lilith the best overall because she's the most fun to play in my opinion.
In terms of best to worst:
PvM: Roland-Lilith-Brick-Mordecai
PvP: Mordecai-Lilith-Brick-Roland