MA is not highest single target damage, that's an urban myth. The title belongs to either BS for highest burst or Claws/DM consistant damage (they're very close with hasten included, and DM can pretty much claim highest burst damage if soul drain hits 7 mobs, and activates damage god mode for 30 seconds)
DM has few true attacks, with fears, a powerful buff, and endurance recovery, and a self heal to compensate. It's the only primary I really enjoy, since the rest feel so bland. Generic damage attacks, etc etc.
Martial Arts has good secondary effects to help take down bosses, which is (I guess) where the reputation as boss killer comes from.
Spines/regen is, due to toxic damage, slow, ranged attacks, AoEs, damage auras and no downtime, the best PVE and probably PVP scrapper.
hmm, with that said I'll narow it down to either spines or dark melee, any suggestions?
Mass AoE massacring: spines/regen.
Being unable to be killed in a 1v1 thanks to massive debuffs: DM/regen.
For a PVE build, I would say spines is the best bet.
I like DM, but DM is very much a single target killer and it can really get to you if you're soloing and wind up with a "defeat all" mission, cause you cant easily gather a group for the Soul Drain. if you team a lot though DM is great, normally more than enough foes around to hit that god mode, the lovely debuffs for AVs that your team will not know you've done and give you no credit for, and just generally the single target killer of the team. On the other hand you can herd with spines nicely, its got a good mix of powers and a great ability to grab and hold agro. As a regen agro means nothing to you
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
hmm, after much thinking and the replys I think I'll go with spines/regen. I'll psot a build whenever I can be arsed.
I consider MA as the most balanced scrapper choice because it combines good damage with fast recharging attacks and an ability to control using stuns/knockdowns as well as relatively low endurance.
It will not however give you the highest damage (broadsword) a less resisted damage type (Dark Melee very awesome) or much AoE fun (Spines best, others have more cones and stuff as well).
I personally don't rate claws or katana as quite so good but it's not exactly a huge gap.
aww damn it, so many choices yet so little time.
From a purely romanticised point of view, Claws feels the best out of the Scrapper primaries. It's fast, the end cost is unnoticeable - in short, the most scrappy Scrapper power, in my opinion.
If you prefer effectiveness over style, don't pick Claws 
. But if you want to feel like a real Scrapper, try them. They can be frustrating at first, due to the low-ish damage (if you're teaming with a ST Blaster, you'll notice the difference), but as you progress and sort your slots out, they've got a lot going for them.
Especially (I've only noticed recently) in PvP if you're lucky enough to go up against someone with no knockback protection. With Hasten and the quick recharges, Focus and Shockwave can keep your opponent flat on its back for half the match, allowing you to take the time needed to drain its health 
.
What no Broadsword? 
BS/Regen is pretty Uber!
Defiant 50's
Many and varied!
@Miss Chief
OK, I am mainly a fan of city of villains rather than city of heroes, but I jsut love the way scrappers tear through thier enemies after a certain level. I want to make one for myself, I already intend to pick regeneration as my secondary wpoer set but I'm not so sure on the primary power set, so here are the contestants.
Dark melee : most likely pick this one
Claws : Quick and low end
Martial Arts : High damage
Spines : least likely to pick this, but still cool
Please help me pick!
EDIT: btw this will be mainly a PvE build.