Hearthpwn

C'thun decks being above tier 4 in this day and age is really debatable. 

Back in the day combo cthun druid and cthun warrior were your best bets. Right now I think you should go for cthun warrior since druid has been gutted.

The best c'thun cards (besides the two which you mentioned already) are Disciple of C'Thun C'Thun's Chosen, Beckoner of Evil, Ancient Shieldbearer and a single copy of Doomcaller.

The only C'Thun deck I am playing contains Shudderwock also. Idea is to buff SW with battlecries such as Omega Defender and that 1/4 dragon that gains attack for each card in your hand. Than hopefully C'Thuns battlecry triggers after it is buffed dealing lots of damage.

Warrior should be the best right now. Abuse Dead Man's Hand to get infinite C'thuns.

Alternatively, Druid should be fun as well. Use Juicy Psychmelon to draw C'hun, Twin Emperors and the C'thun Revive guy. However with all recent Druid nerfs, I think the deck should feel very clunky.

The most difficult to make is certainly Warlock. It's C'thun support card suggests a token-based playstyle, however, C'thun decks lean more towards Control archetypes because C'thun is 10 Mana.

However, I don't think any of the C'thun decks is even remotely competitive right now. Fun, sure. But even the T4 decks that Tempostorm lists are a lot stronger.

There are no decks coming up in HSReplay with C'Thun, meaning there is no C'Thun-deck that at least 200 people played last 30 days. 

I played a fair bit of C'Thun Rogue when it was still in Standard. Very enjoyable deck.

Are you talking "viable" as in fun or as in "legend viable"?  I also got the Wild bundle, but I didn't get the Twins (only Shifter Zerus from the OG packs).

I've been bored with Standard lately and tend to move to C'Thun decks in Wild because he's just so fun!

Shaman can be fun with a C'Thun + Shudderwock combo.  You either use Shudder to double up on your C'Thun battlecry buffs, OR you play C'Thun first, then with other battlecries like Fireguard Destroyer and Omega Defender you hope Shudder gets attack buffs before the C'Thun battlecry goes off.

I was trying to make a Rogue C'Thun deck work, simply because I wanted to play with Blade of C'Thun for some reason.  I wouldn't suggest it.

Druid is still good, but it takes times to pull off the big combo.  I'm running the Aviana -> Kun the Forgotten King -> Brann Bronzebeard -> C'Thun -> Zola the Gorgon on C'Thun -> 2x golden C'Thun combo, but with no Wild Growth and one Nourish (mostly for added card draw).  My list is likely not the most optimal for this deck, but I rarely play Wild and I've climbed some of the low ranks with it.  (https://www.hearthpwn.com/decks/1227656-nightmare-druid)

 I have been very careful to include any new cards from the latest expansion to add some win more and card draw to my latest creation and I must say i am very pleased with the outcome and this is deffinitely the best C'thun deck I have ever made and seen as the mixture of both N'zoth and C'thun in priest topples most regular control decks as they just can't deal with the damage the only exception being dead man's hand warrior which is neigh extinct.

https://www.hearthpwn.com/decks/1284588-the-old-gods-mixtures

Shaman Is fun. 

 I am currently playing priest N'zoth

And buff paladin N'zoth + C'thun decks

I'm playing C'Thun in my Warlock.

C'Thun is so great with Cruel Dinomancer and Expired Merchant.

play shaman with quest

Probably Shaman cuz you can do some shenanigans with the quest and Shudderwock.