Morandir

Bah! I knew I'd get that wrong going from memory. That'll teach me.

Yeah you are right - it's Claws, Dual Blades, Katana, and Spines (from Mid's).

I'm listing everything rated best to worst (in my opinion of course) based on your criteria.

For pure AoE effects, Scrapper has the following:

Electric Melee: 1 Cone, 2 PBAoE (1 non-damaging), 1 multi-mob attack (Chain Induction)
Spines: 2 Cones, 2 PBAoE
Dark Melee: 1 Cone, 2 PBAoE
Claws: 2 cones, 1 PBAoE
Broad Sword/Fiery Melee/Katana/Kinetic Melee: 1 Cone, 1 PBAoE
Dual Blades: 2 Cones
Martial Arts: 1 PBAoE

Electric Melee has the most AoE attacks. It does damage that isn't resisted much by mobs. Spines is also a very good set in this respect. I give EM the edge because cone attacks can be a pain - Chain Induction is an auto attack once you've fired it off. In a large group of mobs it will hit quite a few of them.

For Secondaries, if you are looking for damage:
Dark Armor, Electric Armor, Fiery Aura: Damage Aura
Shield Defense: 1 PBAoE (a nice one though)

Any of the above will work well for damage output. Shield has a damage buff power which is nice and will pair with your attack set's buff as well.

The downside to Shield Defense is that you cannot take it with some of the attack sets - Dark Melee and Dual Blades are excluded. There may be others I can't think of off-hand.

Having said all this, none of the primary or secondary sets for scrapper are *bad*. What you will consider effective will depend on your goals.

For pure damage output I'd make an Electric Melee/Shield, but I think that's been done to death (for good reason). Spines/Dark is nasty too - I've played one of those to the mid-20's and killed loads of mobs in cranked-up missions.
I also have a Fire/Dark that's very effective against groups.

The issue you will run into is that these scrappers tend to be squishier than a WillPower, Super Reflexes, Invulnerability, or Regen scrapper at first. I think they all have built-in heals for that reason.

Anyway, good luck and have fun!