If you are just looking for single slotted powers any of those should be fine. Recall friend and mutation are a nice choice for helping teammates out and neither require extra slots. SI invis is a better stealth option if you are just using a stealth IO in rush, yes. If you don't plan on running it 24/7 it's also a good mule for a LotG +recharge IO. If you don't have a travel I guess SJ would be nice.
@Mental Maden @Maden Mental
"....you are now tackle free for life."-ShoNuff
First, I suggest you take a look at my Illusion/Rad guide, linked in my sig. That may answer some of your questions.
As far as Invisibility: The game shows full invisibility as 70 ft perception, which is the stealth cap. There are really two "levels" of stealth (there are actually a few more). Powers like Stealth (from the power pool), Super Speed (power pool), Steamy Mist (Storm), Shadowfall (Dark Miasma), Shadow Cloak (Warshades) and several others are basically half invisibility. The will keep you unseen from a distance, but not if you go right up to a foe. The Stealth IOs like Unbounded Leap are also half invisibility. These are all shown as 35 ft perception
To get full invisibility, you have to stack two halves, but some of them won't stack. Usually Super Speed will stack with most others. Usually, the Stealth IO will stack with most others. Or you can use one of the full invisibility powers like Group Invis or Superior Invis. Superior Invis actually goes way over the stealth cap, but that extra is wasted except in a very few situations where there are foes who have higher perception . . . in those cases, SI is better than GI or two half-stealth powers stacked.
As for the practical application -- half stealth will let you sneak by foes as long as you stay at a distance or go by really fast. If you run into a room where foes are blocking the door, then you are likely to be seen. Full invisibility will let you stand in the middle of a group of foes dancing and they still won't see you.
My guide goes into a discussion about SI vs. GI, but I recommend taking one or the other unless you prefer to have Super Speed + a Stealth IO. Both SI and GI will hold a LotG Recharge IO to help with your high-recharge build.
There are a lot of one-slot-wonder powers that can be useful. I really like having Recall Friend. An Ill/Rad with SI is the best team spy around, and Recall Friend makes him into an even better spy. (Plus you can slot a Zephyr -Knockback in the default slot).
If you went with the Fire APP, Rise of the Phoenix is the only self-rez available to controllers, and it only needs one slot -- Recharge or Damage, depending on how often you think you will need it. I'm not a big fan of the Psi pool for an Ill/Rad, and my guide explains why. From the Psi pool, Indomidible Will is the main reason to take the set, and it needs 3 Recharge. Psi Blast should be in your build (if you stick to the Psi Pool) for the great Blind-SW-Blast-SW attack chain. Psi Tornado is a decent AoE power and Mind Over Body is a decent resistance-based armor. World of Confusion, however, absolutely stinks.
I like having Combat Jumping and Hover -- both take a LotG Recharge. Combat Jumping provides protection from Immobilization powers, jumping mobility and a place for a LotG Recharge and uses very little endurance for a toggle. Hover lets you fight from the air, out of melee range.
Mutation is a nice Rez power that gives a huge boost to the rezzed teammate. I currently have it on my build, but I wouldn't mind dropping it if needed. There are so many ways to rez folks in the game today that it is nice to have but far from necessary.
I haven't tried the 5th power from the speed pool yet, but I can see its appeal, especially if you don't have perma-PA yet. It could be very useful on a Mind or Plant controller or other powerset combos that have long recharge powers that are not persistant. I'm not sure how much it is needed for an Ill/Rad, though, but I haven't tried it.
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Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
I'd vote on Recall and Mutation too, I know that I always wish I have it on toons without it and use it all the time on toons that do.
Alternatively Burnout is nice for burst damage on Ill/Rad since spectral wounds' damage tends to goaway if you don't kill fast enough.
Hi all
cracking info and positive thinking which has sorted me out.
My alt is actually based on your guide LM - respect
I had him at 50 before the Inherent changes and have just dusted him off after a year or two.
I am going the Recall Friend/Superior Invisibility and Mutation route. I feel this gives me an even more rounded team support member whilst improving my solo stealth ninja stuff as well.
Really appreciate the fast and very knowledgeable wise heads responses.
Thank you all.
So many cats - So few recipes!
Age is of no importance,
unless you are a cheese!
I ended up taking both invisibility powers on my ill/rad. They have their own uses at times and both just go with default slot with a LotG in each one. 
Ideon's Paragonwiki page
Member of Paragon/Rogue Knights
Arc: 60092 - Supa Rumble in the Park
"Keep living the dream, and never let any jerk tell you what to do."
-- High-Roller
Hi Guys,
am about to respec my ILL/Rad to get the benefit of inherent fitness.
It is about as purpled out as it can and so am really happy with my base build.
I am trying to work out what 3 power picks I can take when Fitness pool is gone.
I have no spare slots so will be taking powers that are okay out of the box.
I am looking at (in no particular order of importance):
Superior Invisibility from the Ill set
Recall Friend from the TP pool
and then one more from the following - Mutation or Super Leap or the new 5th power from speed pool or another power from the Psionic Mastery pool.
I have an unbounded leap stealth slotted in my prestige power rush at the moment so I wonder if the Superior Invis is better for stealthing? The Unbounded Leap stealth would then be redundant I think.
Is Mutation any good?
Any thoughts or alternative plans will be most welcome. Thanks
So many cats - So few recipes!
Age is of no importance,
unless you are a cheese!