Just to add to the consensus. I wouldn't consider Stokt unless you've got more money than sense.
Of the free apps, I would order them Retro Flash > Eat Spray Climb > Open Climb. I've now moved on to using Chroma to go with my LEDs, but I don't think that's intended for general release yet.
They all do the job and they all have their quirks and annoyances but are great value for your £0.
Some advantages to Retroflash:
1) If, like me, you forget beta quickly, the ability to save beta videos can be very helpful. Saves a lot of wasted attempts in power endurance sessions
2) If you struggle with setting problems, you can ask a mate to set problems for you through the app. Can be good if you always end up setting the same style problems.
3) You can group problems in to folders for quick navigation to Projects, Warm ups, Power Endurance problems etc.
4) You can filter problems by style if you want to work on a particular type of problem.
It has some social aspects which you may like or hate. They're easily ignored if they aren't for you.
+1 to using Boulder Bot too if you're struggling for inspiration when setting problems. A lot of what it comes up with is log but you can quickly scroll through until it creates something decent, and then enter that in to whatever app you use. Its success rate is low but it's come up with some bangers that I never would have tried setting myself. I have to tweak every start move though, it doesn't seem to understand a board without a kickboard so it overestimates how high I can reach from a sitstart and how low I can reach with my feet after sticking the first move.