What a question!
Engineer is probably what you are looking for, they are probably the strongest overall (most options) as well as having the best aoe DPS and one of the best single target DPS (behind thief). Warrior may or may not be in 2nd place after dec10.
Guardian- typically bunkers, they ensure capture points do not get lost as well as taking care of things like stability stomps (finishing a downed player) and revives. They also provide some healing/boons and condition removal to their teams. They CAN spec DPS but will generally have a rough time of it because of their poor mobility.
Warrior- currently in the meta we have turtles and team fighters. The turtles just have very high health regen plus armor and invulnerability skills making them very difficult to down. They are probably a bit tougher than guardians and while they do not have the same support capabilities, they have a much better elite skill (instant revive banner).
Team warriors generally bring tons of aoe and crowd control while still having the benefit of high hp and defensive moves that make them an unappealing target for focus fire.
Meta Rangers are much like turtle warriors in that they are very tough to down with great regen plus have great team support by giving their teammates a number of attack procs. They make great home point cappers because they are quite strong in many match ups, and last a long time even in unfavorable ones.
Thieves are almost always playing glass cannon roamers. They show up to a fight, try to burst a player down, and not hang around longer than they are needed. They have a better time as burst because their teleports allow them to escape where other classes would just die. A little pressure can force them to retreat.
Engineers currently fulfill the aoe spam or bunker roles. It is suicidal to be within close proximity of a bombing engineer no matter who you are and their heals are very reliable. When they choose to play bunker they offer a lot of crowd control that can knock players off point, but rely heavily on healing to stay upright.
Necros bring tons of aoe conditions like engineers while having better range and more hp, as well as having great CC and boon removal. They can be anywhere on the range from glass to tank making it very difficult to know if they should be focus fired or not. They also have plague form to allow them to survive even under impossible circumstances.
Elementalists generally are not played, but can have good single target and aoe damage. They are very difficult to play and have limited escapes. The new patch may open things up for them in terms of builds (condi and bunker). For a long time they were the goto for team support bunkers with condi removal and are itching to make a comeback
Mesmers fill a variety of roles. They have specs that are very strong vs typical 1v1 builds but are typically fragile. Their survival depends on the cool downs for their stealths and invulnerabilities which threaten to make focus fire attempts risky to useless. They can burst, crowd control, or sustain but are almost as difficult to play as elementalists; you might just find yourself doing nothing but trying to stay alive while contributing nothing
