Which siege weapon in ESO PVP does the most damage?

Solution 1:

I experimented with the siege engines on resource towers and on the dummies on the training field, and got some numbers about them:

Balistas

• Balista
  2000 to structures
  270 to Units
• Fire Balista
  1000 to Structures
  1080 to Units over 3 seconds (360 per second)
• Lightning Balista
  1000 to Structures
  810 to Units over 3 seconds (270 per second) and stun

Catapults

• Scattershot Catapult
  2000 to Structures
  180 to Units
• Meatbag Catapult
  500 to Structures
  540 to Units over 3 seconds (180 per second)
• Oil Catapult
  500 to Structures
  451 to Units and ensnare

Trebuchets

• Stone Trebutchet
  2500 to Structures
  360 to Units
• Iceball Trebutchet
  500 to Structures
  225 to Units and ensnare
• Firepot Trebutchet
  2000 to Structure
  higher than 2400 to Units. The first dot destroys a 800 hp dummy, so I couldn't really measure it

Battering Rams

• Battering Ram
  It is difficult to measure it, because it may vary between having 3 to 6 players near a ram, and you need an actual enemy keep door to measure it, and it is a mess during battle.

Flaming Oil

• Flaming Oil
  810 to Units over 3 seconds (270 per second)

I didn't have the opportunity to test the siege weapons against other siege weapons in the field.

Solution 2:

I think this is also explained during the campaign introduction/tutorial, but basically you've got five types of siege engines/deployables:

• Ballistas offer the highest single target/small area damage and are most effective against single players and other siege engines.
• Catapults offer the highest area of effect damage as well as extra effects such as snares (also available on other siege engines, but less effective due to smaller area).
• Trebuchets offer the highest damage against structures such as walls, gateways, etc.
• Rams may only be used against keep doors for obvious reasons.
• While technically not siege engines, forward camps are purchased and placed just like siege engines and allow a limited number of players to use them as wayshrines when respawning.