Hearthpwn

topic.  I want to get a card back,  It I don’t know which one to attempt first.  I hear Naxxramas is a nightmare on heroic.

Karazhan

what decks should i use going for the karazhan (i have no wild cards and no other adventures)

I believe you'll have way more fun building your own decks according to each fight rather than netdecking.

http://www.hearthpwn.com/guides/1548-one-night-in-karazhan-a-hearthstone-adventure

There are guides, and decks for each bosses. Keep in mind, that Heroic Malchezaar is a pain in the ass without Kel'Thuzad.

Karazhan is by far the easiest one but with all the cards available right now even Naxx or BRM that used to be hard shouldn't be a problem.... Sure,if Maloriak has Release the Aberrations! on curve you might has well restart and the same can be said for all those bosses that need you to mulligan for specific cards or for fights that are heavily RNG reliant (Like Skelesaur from LOE) but beside that you shouldn't have too many problems.... If I had to pick one I'd go either Naxx cause I find Kel'thuzad really entertaining throughout the whole adventure  or BRM cause I find the cardback to be the coolest looking one....  

Karazhan is laughably easy with the exception of Malchezaar. Took me a few tries but I got lucky with a Rotface deck that summoned Paletress, and I kept using Hero Power after that to get strong minions.

Naxx Heroic is probably the hardest. Blackrock Mountain had some easy ones but a lot of them were brutal. Explorers League was second easiest I found, the hardest was Skelesaurus Hex because that Heroic battle is nothing but RNG. Nothing like having your opponent play Varian on Turn 1.

Probably Karazhan but the big concern is the chess challenge.  I found it very aggravating as there is no deck building process.

Karazhan is the easyest, I think. The final boss is hard, but some of the newer cards might help, especially things like infinite combos (quest mage, questless exodia, etv).

Explores is not that hard too.

The main point is the access to cards and/or combos outside regular ladder play. If you share your collection it makes it easyer for people to help you.

Shappiron will be the hardest. Freeze mage works well. But will take a few tries. 

Kharazan is the easiest in heroic, the only tough battle where you need something really specific is the final boss, the rest of heroic Kharazan can be done with Meta decks or decks that were meta at some point, i presonally enjoy playing Patron warrior against the boss that summons the plates from time to time.

edit: i also remembered, Heroic chess can be challenging as well. but that encounter is fun AF and lit AF

Ranked from hardest to easiest:

BRM - Chromaggus sucks! 17 bosses, the most of all the adventures.

Naxx - All the new cards make this easier, second most bosses.

LOE - Just slightly harder than Karazan, one extra boss more.

Karazan - By far, the easiest!  :)  

Of course they all have their fair share of required RNG, but I was able to complete all 4 in a matter of weeks.  Good luck!