I was asked to post my engineer build so here I go. This build was primarily made for wvw as it gives lots of AOE and knockdown abilities. Traits emphasize explosive damage.
Basic weapon should be rifle. Pistol is awful in pvp imo. Your goal with the rifle should be to immobilize with the net gun, and the follow up with a point-blank blunderbuss followed by an overcharged shot to knock your opponent down. You will always get up first after an overcharged shot, so you can get in a jump shot before they get a chance to move.
In large battles, I tend to look for knots of caster/ranged enemies, and either charge in, or jump shot into them. In rapid succession, drop a big ol bomb and a grenade barrage. I usually am getting hit pretty hard at this point so i switch to flamethrower and pop a smoke vent and try to escape. If you healthy enough to keep up the attack, switch to flamethrower. Flamethrower has lots of nice condition damage and CC options. The number 1 attack on flamethrower has a very wide cone, so even though the damage is low, you can easily hit 2-3 enemies at a time.
Traits:
Explosives 20 - Forceful explosives & Explosive powder.
Firearms 30 - Rifled barrels, Go for the Eyes, and Juggernaut. The first 2 abiliites can be changed depending on your play style. I love Juggernaut because it pretty much keeps 6 stacks of might on you permanently.
Alchemy 10 - Self-regulating defenses is a must. This one ability has saved my life numerous times.
Tools 10 - Speedy kits is also a necessity. By swapping between rifle and flamethrower, I can keep up swiftness essentially permanently.
Kits:
Healing Turret for AOE heals.
Bomb kit for AOE damage and the amazing Big ol Bomb. The Big ol Bomb does insane damage AND knocks down, so you want to be looking for opportunities to get close to a big group of enemies and drop one in the center of them.
Flamethrower for Juggernaut (above) and for AOE attacks when you are facing a big group. Air Blast is another knock back with is great for launching 2-3 invaders off a cliff at once. (hehehe)
Grenade kit for siege. Grenades are great for keeping invaders off walls while your team is banging on the gates.
Supply crate for elite skill. I am not particularly fond of any of the elite skills. I often switch to mortar when I am involved in more sieges than open-field battles. The nice thing about it is you can build the mortar and then let someone with poor aoe use it when you throw grenades.