Hearthpwn

well..... first card to pick is a legendary... which one would you pick? 

Gormok.  Most likely to get value and is the most reasonably stated if you play it naked.

Gormok all the way.

I would pick krul because you can draft your deck to make it without dupes as this is your first draft. Krul can get value with just a few good demons, have crazy combo potential with voidcaller and doomguard. Gormuk is good if you can draft cards like implosion or forbidden ritual. But it's still hard to make 4 minions stick on your board. as boardclears are everywhere these days.

I choose Gormok, u can play it early or late game u can combo with some summon minions. Also at arena low mana minion so important I think.

Yup, it's Gormok mate, as you're playing with Warlock.

Drafted him twice in a warlock deck, worked very well for me in both instances!

Id pick the boogeymonster, prolly the least situational to trigger...

Gormok and draft for low curve.

Drafting aggroish deck is more probable than high demon density. Krul would probably pull out 1 demon tops because it's slim chance you'd be hoarding demons in your hand till turn 9 even if you had them.

Boogeymonster is almost playable in arena, but quite slow and worse than others.

Gormok will be worst than a Chillwind Yeti most of the time, The Boogeymonster is bad as we all know and maybe but just maybe, you're lucky enough to pull some value from Krul and probably isn't worth it to try to build an arena deck around him. Sadly, those aren't from the good ones.

I'd pick Gormok.  Most likely to be playable on curve (4/4 isn't horrible) and if you can proc the battlecry, that's just gravy.  Krul and boogeymonster are just too slow.  Either Krul is a dead card until late game, or you're top-decking and have played your other demons out of your hand (if you have any).

Plus, there are a lot of really good warlock cards with the new expansion that will show up frequently with the occurrence bonus.  Why force yourself to choose only one of each of those cards by drafting Krul?

Pick Gormok and choose other cards Zoo likely.