For the plant which makes me happiest erigeron karvinskianus, the Mexican daisy, must be a contender.
We visited her garden one quiet hot day, got talking, stayed for coffee and ended up spending the best part of the day there. Describing this modest little daisy she said 'It sprinkles about'. And it does. Delightfully. Here it is sprinkling down our front steps. The colours of the flowers change from white to pink to brownish pink as the plant matures so you have all these different shades together at any one time (this doesn't really show in my photograph).
I don't have ideal conditions for it - it prefers dry and limey, and we are wet and acidic, but it survives most winters under the eaves of the house in gravel. It's very easy to keep going from seed and cuttings.
Happiness rating 10 out of 10..
...do you have any 10 out of 10's?