Hearthpwn

Runaway Blackwing VS Onyxia the Broodmother on Big Warrior.

Which one do you think it's best? And why?

Take into consideration Commencement!

Runaway Blackwing 100% as it is a lot more proactive (kills a random enemy minion at the end of turn). Onyxia on the other hand has a very mediocre effect (some 1/1s that have no taunt or rush or anything to protect you) and can board-lock you preventing you for example from following up your t7 Commencement with a t8 Troublemaker

Either way, if they survive, the game is probably over, so you have to compare the immediate effect the turn you play it :

What's better : 6 (maximum) 1/1s or dealing 9 damage to a random (allegedly, usualy you can hit what you want with it) enemy minion ?

Runaway Blackwing is almost always the right answer.

They are both really good in different decks.  I would say Runaway Blackwing is stronger because it is has a stronger immediate effect and can also be combined with lifesteal effects in Priest and Paladin.  I think people are slightly sleeping on Onyxia though.  It's nice to combine with another of my favourite underrated cards of the Barrens, Crossroads Watch Post.

There are pros and cons for both, depending on the matchup/situation, but if you play Barricade in your big warrior list, that's a pretty good argument against Onyxia.

Also, as someone pointed out, don't lock your board with 1/1s.