I like this topic so much.
My favorite plant, which was indeed my first Observation, I have told the story of, and given that I was concerned about it at first, it feels lovely and unexpected that now Hamelia patens is my favorite plant. (Sadly H. patens is not in our new garden and I often think about finding it and grabbing its fruits to carry home and fling into the garden. I have not and likely will not but I am hopeful the zanates will do it for me.)
But my heart sings loudest for the tiny, industrious Nannotrigona perilampoides. I know I ought not to, but I identify with this bee in many ways: it is industrious, it is non-stinging, is productive (honey!), it is not picky and appreciates many native plants. I like its neat, nonshowy appearance, its jaunty leg, and that it generously allows me to take photos. I like that I can often guess which flower a particular bee prefers by the color of pollen it is carrying. I like that it makes its home in pipe structures behind holes. I especially like watching them clean their antennae, though I could not say why. I also love when their back legs are so full that they appear to be wearing small pantaloons. In a word, I am smitten. 
I do not know that I would ever have found N. perilampoides were it not for Hamelia patens, because the main plant N. perilampoides visited in my garden was the ever changing, always blooming plant that started it all. And given how small and unobtrusive this small bee is, I likely would never have noticed it were I not looking at the large, showy plant. Now when I am out and about, I see “my” bee everywhere, though I do not observe it, choosing instead to just watch and enjoy and tell those with me what I am seeing and what to look for. It makes me very happy to know that I have saved numerous nest entrances from being sealed just by pointing them out.