My (not so) humble opinion:
Best looks; decent tech; decent app: Hypervolt
Best tech; best app; uninspiring looks: Zappi
Decent looks; decent tech; works with variable charging tariffs: Ohme
Oh, and if you're using a 13A (or 10A) plug-in charger plug it in to something with a thermal cutout. And if you're using it on an extension reel fully unwind the reel!
NB: According to folks in another thread Octopus Intelligent will support Zappi later this year, but personally I try not to place much value on promised features without a firm release date ("end of May" is still pretty vague)