Apple - What's a good iPhone running app with a heart rate monitor?

There is Digifit which is software that works with any ANT+ enabled devices by brand names such as Adidas, Garmin, StarTrac and Timex.

Runtastic has a solution for heart-rate sensor bands communicating with the Runtastic apps (which are similar to RunKeeper) via a sensor key which is attached to the iPhone: http://runtastic.com/en/sensors

The dongle is as expensive as for the Digifit, but the app is a lot more affordable.

Wahoo Fitness

You might like to check out Wahoo Fitness - it's also an Ant+ dongle like Digifit, but unlike Digifit it works with a whole different ecosystem of apps (not just the default Digifit one) - defintely the widest range of all the apps you've listed here, and including Runkeeper as well so you don't need to change which running app you use.

Good luck, and happy running.