I like harvest and natural cure, as they will both have interesting applications to our cap, however, Chlorophyll seems not too useful, as one of our main goals is to stop rain abusers, and if we can't function in rain, its seems pointless. And Flower gift just seems to skew it for the stats stage.
We decided to make this thing a sun based pokemon, and that's never going to happen without a sun based ability. Natural cure doesn't approach the concept in any other way either, really, so I think it's a generally poor fit. Flower Gift is my preferred option, as it is simple and predictable, tying our CAP to sun without concerns of brokenness. With harvest it's really tricky to know how strong to make this CAP in other departments. Nonetheless, it could be very effective if we're careful.