Weapons I’ve tried and had success with in WvW:
Hammer – good for defense (holding bridges, wall fights, stairs, and gates). Mighty blow hits HARD and often. The immobilize helps you secure kills. Banish is great for stopping stomps/rez, knocking off ledges, and just plain fun. Ring of Warding is great for area denial and trapping players (for a second at least…)
Greatsword – offense. It allows you to leap into a zerg, pull, and quickly deal massive damage. IMO Greatsword > Hammer for lifesteal builds since you hit more often.
Staff – defense/support. Again you can block gates, bridges, stairs, and walls with Line of Warding which is MAJOR in a battle.
Scepter – your best ranged option because it has an immobilize. Smite works wonders for lifesteal builds also. Don’t underestimate Scepter, easily a 1.4k auto-attack crit.
Shield – group support. Protection is nice to have for your team, but Shield of Absorption is where it’s at. Knock enemies off walls, block an enemy’s burst, heal allies, interrupt heals, and prevent stomps (this is major in a zerg fight!)
Focus – defense. Eh, mainly self-defense weapon. Allows you to stay in a fight a lot longer. Shield of Wrath is great against CnD thieves… they usually waste all their initiative on your shield, then you can impose your will on them.
The general combo I use is Greatsword + Scepter/Shield. For defense I use Hammer/Staff, but it depends on your build I suppose.
As fishergrip noted earlier, the GS leap will save you. It allows you to immediately place yourself in the fight. Rushing with a hammer doesn’t work most of the times… about half of your health is already gone by the time you start fighting. The same reason is why I always carry a ranged weapon – Guardians are tough, but you can’t stay in melee range forever. You need a way to retreat and recover while still assisting your team.
I too can wreck thieves while wearing greens using Moa.