Which Adobe software is best for iphone apps

The best software for developing iPhone apps would be XCode. If you can, always go native. AIR has lots of drawbacks and for me (after almost two years of cross platform development with AIR now), the benefit of addressing two platforms doesn't outweigh the drawbacks.

You're specifically asking for iPhone, which sounds like your only target, so I'd stronly advice to go native.

It might be worth explaining more about what the apps would do. If they are to be brochures for the agency, maybe you could use Dreamweaver or InDesign, and be a Creative Cloud customer, and use the PhoneGap exporter. I haven't done that, so I'm not sure what's involved.

One difficulty though is that if you want to submit such an app to the iTunes App Store, it may get rejected. Apple don't accept apps that are just promotion apps that could easily have been done as a web site.

If you want to do apps that are more interactive, or game-like, then Flash Pro would be a good option. That can publish to both iOS and Android, and it's what most of us here use (though some use Flash Builder instead of Flash Pro to do the development part).