First, AppGameKit is the closest to DBPro you will find. The Syntax is the same, just the commands are different. It's pretty easy to learn.
But from my experience of those you mentioned:
Unity, steep learning curve, 3d only which puts me off straight away (although I believe the latest version can do 2d...)
Gamesalad, Urghh! Hate that one
nuff said
Stencyl, it's good, but very simple
Out of those 3, I'd go for Stencyl
Also, it's important to say that it's not possible to publish for *free* on iOS, there's the $99 developer fee...
Put simple, my opinion is that AppGameKit is
the best coding out there. But it's got it's problems (a bit buggy, rather limited set of commands, difficult to compile for other platforms)
The most polished one has to be GameMaker::Studio, but it's expensive. Fortunately it manages to combine simplicity and power (if you're willing to learn GM script). It's main problem is that it's so easy you probably won't put the time into using it to it's maximum potential.
Construct 2 is pretty good as well, but still limited.
EDIT: Oh yeah, and there's monkey. If haven't tried it personally though, but many people say it's good
