It really depends a lot on what you want to be able to do, and how much time/money you're willing to spend on a build. First up, in case you haven't found them yet, there's nice list of what the various sets do here, and the legendary Mids' Hero Designer will make organizing a build much easier.
Anyway, I believe the most powerful options are generally focused on positional defenses (Melee/Ranged/AoE) and Recharge. But most of the sets that can add those are expensive to obtain. Just remember that you can only benefit from a maximum of 5 copies of a given bonus (so 6 +1% Damage boni is a a waste, but 5 +1% and a single +3% would work just fine).
Personally, I settled for cheaper IOs and focused on +Recovery because I tend to solo a lot (so no help from Vigilence, even by its usual standards). I also added a bunch of Energy Defense, since it felt thematic (and also grants second rate Ranged Defense now, which is actually helpful). I would recommend getting a -Knockback enhancement, though: it's nice not to be bounced around all the time. You can get one from Steadfast Protection (Resistence, also has a very nice +3% Defense IO) or Karma IOs and slot them in Shadowfall, or you can get the Blessing of the Zephyr one and slot it in a (non-Sprint type) travel power, but that's generally a lot more expensive to obtain. You can even slot multiple copies of them for more Knockback protection, but I found a single IO prevent enough knockdowns for my tastes.
Also, bear in mind that you may be able to achieve more by combining IOs from multiple sets in a given power than you could by slotting all of 1 set. For example, I combined 3 Thunderstrike, 2 Stupify and a Dam/Mez Hami-O to get a lot of recharge and damage into Cosmic Burst, while also increasing its Stun duration and getting nice Recovery and Energy Defense set bonuses too.
-D
Darkonne: Pinnacle's (unofficially) mighty Dark Miasma/Radiation Blast enthusiast!
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#161865 - Aeon's Nemesis
Just returned to the game after a hiatus, have a lvl 45 dark/rad/dark defender. Here's the thing - im new to the whole IO sets and was curious if anyone has recommendations for IO slotting. Which Sets benefit the build appropriately? Obviously the hero is not a PvP/Solo type Def, but more double debuff/group support def.
any help is greatly appreciated!