Personally I like the Serendipty set in Focused Senses / Fighting...the bonuses are pretty good for SR, though you'll need to 4 or 5 slot powers that can do pretty well with just 3 slots. It's not too expensive, either.
Powers like agile you can slot with a defense IO and forget about for a while; and I found that with Practiced Brawler (3 slotted for recharge), I could get it permanent at 20 thanks to Quickness even using IOs I'd bought a while back.
I guess that is not an easy question to answer, since the effects of sets are to be looked at in the totality of the powers, you can get more defense from a melee set and more dmg from a damage resistance set.
Ideally, what you want to boost is almost everything, defense, recovery, regeneration, damage, accuracy and recharge reduction.
The sets I use on my SR scrappers usually are:
Numina's Convalescence Unique Io (+regen +recovery)
Miracle Unique (+recovery)
Luck of the Gambler (especially the rchg bonus one)
Red Fortune
Mako's Bite
Touch of Death
Scirocco's Dervish
Steadfast Protection Unique (+3% defense)
Multi-strike
On the other hand, these are probably the most expensive sets nowadays.-
Regarding regular IO's, I try to wait until lvl 32 (not always possible, but at least until very late 20's) to equip lvl 35 regular IO's, but if i get any unique such as Miracle or Numina, I equip them asap since the bonus is same regardless of lvl.-
You may want to take a look here to some build I posted to get some ideas for your own builds: Clicky
for the toggles I use 5 slots.
Luck of the gambler set:
Defence
Defence / Endurance
Defence / Endurance / Recharge
A standard Defence IO
A standard Endurance IO
This gives 3x 10% regen and 3x 1.13% extra health as well. Combined with 2x 10% regen from using 2 sets of Scirocco's Dervish on 2 of my Katana powers, it gives a decent bonus 
For practiced brawler, its 3 x defence IO, 2 x endurance.
With quickness as another power it's perma with a good stack on it for when it recharges!
In-game and now on Twitter @Tsumiju Zero "The Nightmare of Dra'Gon"
"The flow of battle can only be influenced, not by realtime tactics, but by strategy."
Proud resident of the Union EU Server.
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ty for the help guys i forgot to ask as well, if you combine Defence IOs with standard Defence IOs or SOs, does the normal 3 slot rule work as normal? i.e if i have 3 Unique Defence IOs in and put another 1 or 2 standard or SOs in, does the %bonus go down in the same way as normal?
Yeah it goes down as normally. It's a question about the percentage, not the enhancements. ED hits in (for DEF/RES) at around 50%, so 2x IO + 1 SO will easily get you to maximum, 2 IOs is nearly there. 3 IOs + 2 SOs is only a few percent better than 3 IOs.
- @DSorrow - alts on Union and Freedom mostly -
Currently playing as Castigation on Freedom
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I've gone with Red Fortune in active defences as I had them slotted 3 DEF and 1 ENDRED SOs (4 of Red Fortune gets you the set's damage bonus and Flitz has always been a little low on dmg output). I will possibly respec to get the full RF set in Focused Senses for the extra recharge and ranged def bonus. Red Fortune is also cheap to buy (or was when I did it) and def is about the same as it was with SOs.
I've gone for Serendipity in the passives which had 3 DEF SOs (3 from the Serendipity set adds some extra regen and much needed HP as I have no self heal). Again not much difference in def between IO and SO slotting.
I'm aiming to put Luck of the Gambler in Elude (mainly for the recharge), but it's very expensive.
Note that PB is 3 RECRED (although 2 is enough) - Tsumiju clearly posting without enough caffeine.
UNION @Flitz 50, Lead Hose 50, Red Rag 50
DEFIANT Rose Bloodthorn 34
VIGILANCE Captain Caledonia 20 - Yeah, I made toons on the French server coz we only had 4 back then (might have to transfer/recreate them on one of the US servers)
..and many more!
I was just looking for some ideas on what the best IOs would be for SR.
I am only LvL15 atm and have so far chosen Focused Fighting/ Focused Senses/ Agile/ Practiced Brawler.
I will be taking the rest of the SR line in turn as i lvl, except LvL16 and 20 which are marked for Health and Stamina.
I would also like to know whether its worth slotting with IOs at my current lvl as probably id be upgrading them quite quickly.