From my experience, you have a couple of secondaries that stand out. Among them:
- Martial Arts, for the awesome, basically constant, +10% to all defenses. Especially on SO builds, they help get the defense sets to the 'softcap' easily, letting you live easily with moderate slotting in the defense toggles. (For heavy IO builds, you likely want to pair it with resistance sets instead, to get both resistances and defenses up there.)
- Super Strength, for the awesome, basically constant (outside the crash) +80% to all damage. Alongside the best AoE attack available (Foot Stomp) and decent enough single-target smashitude, it offers a very solid damaging set.
- Dark Melee, for the handy Siphon Life, offering you a heal for sets that lack it, and the lovely Soul Drain instead of silly old Build Up.
- Ice and Stone Melee, for Ice Patch and Fault respectively. These do very well on teams, letting you play at being a controller even as you tank away, both also having nice capstone powers in Frozen Aura (a smaller Foot Stomp) and Seismic Smash (a reliable hold in a great attack) respectively.
Thanks!
How's electricity/electricity with so's only? Looks like a good combo. Damage aura and lots of end draining for groups.
Really anything will work fine on SO's; after all for the first several years of the game that's all there was and an SO build is just as capable today as it was then.
Also, you can't really single out "this secondary shines with IO's while that secondary doesn't need them" as it really doesn't matter. In a nutshell all secondaries are focused around hitting things and they all need the same things: accuracy, damage, recharge and endurance reduction. Taking the secondary in isolation I really don't see one set having an advantage over another when comparing SO to IO.
Yes any build will become better with intelligent IO selections but you're looking at overall character, not simply the secondary. The primary has far more effect on how the character performs with SO's vs a top IO build; in large part the secondary is irrelevant in the SO/IO debate.
COH has just been murdered by NCSoft. http://www.change.org/petitions/ncso...city-of-heroes
That makes some good sense. 
I think Electric Armor is great with just SO's Its even better with IO's but you have to know how to play it. To stay alive you Gotta Sap End if you only have SO's
As long as I'm asking questions...
What secondaries do particularly well - or poorly - on SOs, both for soloing and for teaming?