I was debating posting exactly this same question; I'm glad you beat me to it. 
On pure SO's (or even with an unlimited budget IO build for that matter) nothing is more survivable than Stone Armor in Granite. Of course you pay for that with loads of penalties so it's hardly ideal if you want to run small team or solo.
Frankly if you're strictly looking at solo then I'd suggest either a scrapper or brute instead of a tank; they have more than enough durability for solo and quite a bit more damage. The tanker's strength in durability and aggro control shine in a team setting but it's lower damage output is a disadvantage in a solo setting.
Getting to your question the Invuln will be more durable on SO's, and with a soft capped IO build it's going to be stronger there as well but the margin is narrower. Invuln is also considerably better at aggro management since it's taunt aura is literally an order of magnitude stronger than WP's. That isn't to say that WP can't tank, it just has to work a lot harder to be almost as good.
WP's advantage comes with it's better endurance tools giving you a second, somewhat stronger, version of Stamina in Quick Recovery. Still, barring considerable IO set investment it's going to lag behind Invuln in durability.
As far as secondaries go Super Strength is quite popular, I haven't played it myself though. Stone Melee is another solid choice with good mitigation and the tanker version of Martial Arts offers a sizeable chunk of defense in it's tier 2.
I've started a Super Reflexes/Martial Arts tanker and I have to say that it's unquestionably the most durable tank I've ever played in the low levels and it looks to be quite competent in the later game as well... for a pure SO build it may be hard to beat. As an example by level 4 I had soft capped my defenses to melee & ranged damage thanks to the +10% boost from Storm Kick. By level 14 I had soft capped all damage positions, something that's nearly unheard of. If you're looking for straight durability on an SO based toon that's a hard combination to beat. It's light on AOE but it has solid single target damage.
All this being said I still suggest a scrapper or brute if you want to solo... the durability advantage a tanker has is overkill solo and you sacrifice damage output to get it. If you want to team then by all means roll a tanker and learn how to manage aggro, teams will love you for it. Solo however you'll move faster as a scrapper or brute.
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Thanks for the info. Well im looking to solo mostly but of course there will be group play also on occasion when I don't have RL aggro. The reason I was leaning towards tanker is cause of the defense first approach. I don't need a ton of dmg I just want to walk up to a group of mobs shake hands and proceed to stomp on their faces lol.
Solo, any primary should do the trick to be honest. A Fire/Stone or Fire/Dark is not only going to get the great AoE damage from the primary, but a ton of mitigation and ST damage from the secondary.
Inv is probably the best on SO's for survivability, although the others will be good as well.
MA (Storm Kick) provides a constant 10% def boost, so that might be of consideration if you decide to go Inv/ or WP/ (SR/ doesn't really need the extra def since it can already soft-cap on SO's.
KM I prefer on Scrappers/Stalkers because of the special mechanics it utilizes with critical strikes.
SS is... SS.
/Dark is excellent for ST damage and survivability.
Another option, is Dual Blades, which I really like. Good ST and AoE damage and mitigation, but not everyone likes it.
Inv/DM is pretty much my best suggestion for survivability + damage. Even on just SO's, the -tohit from attacks and the fast heal from Siphon Life make it stupidly durable.
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My best solo tanker is a fire/fire/pyre. Simply put Fire Armor has plenty of survivability for most if not all solo play even with SOs. Some settings you can even go +0/+6ish and jump into a bunch of mobs, walk away, come back and they are all down.
Do not underestimate the power of fire armor with healing flames on auto-fire. I soloed this guy almost all the way to 50, and still use him for solo farming. With the IOs I have put into him, he's probably more survivable than a generic IO'd Inv tank. I know I have made certain scrappers jealous over my PBAoEs. He literally is like a mini-blaster nuke going off much more reliably and more often, and able to survive it.
If I were to design one today I might go WP/Fire. WP is great for endurance, just keeps going and going with just SOs.
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Hi all returning player from way back in the day. When I played fire tankers could solo a whole zone and laugh while doing it. Oh how things have changed. I been messing around on scrappers and brutes but their lack of surviving a whole lot isn't great. Im sure if you invest crap loads of influence into them and deck them out it would be nice. Im vip for my free month for getting going rogue but will soon be dropping down to premium. So I wont be using any io enhancement in the build. I have been reading many guides and posts on different builds. Stone/ looks awesome for a tank but with all the negatives of granite so's wont help a whole lot. I had a regen scrapper and its way to clicky to survive for me. So that leaves invul, willpower, or super reflexes. They look really nice. Seems invul is better for overall resist and defense but wp has the regen to back it up and reflexes can get defense easily.
Which is easier to get the most survivability out of ,only using so's? Invul,wp,or sr? Im not looking to solo av's but most content before incarnate stuff.
I still want to kill stuff at a decent pace. I was leaning to /ss, /km, /ma is the ones I can think of. I like ss concept but having to wait till late level to get its only aoe is not cool.
Is there a secondary that anyone recommends? Im not a fan of fire or dark(can't remember if dark is a choice).
I want decent damage with great survivability. If any recommends something just give an explanation as to why. And would love any builds you think will work with only using so's. Keeping in mind im gunna solo alot because kid and wife aggro will keep me from playing any long periods of time uninturrupted .