That's a tough decision to make...I'll have to confer with my girlfriend, though we'll have different answers. She loves Poccillopora and I don't!! In fact, she recently made me drive 600 miles round trip, in one day, to buy one. I will admit, it's beautiful. It's about 7" in diameter and split into two colors, right down the middle. The amazing thing is, it's a single colony. Looking at it, you'd think it came from 2 frags glued together.
My favorite would be a bit harder to decide. Give me time to think it over...lol.
Okay, for now, I'm going to have to say it's my purple M. digitata. I know...pretty plain coral. It's probably because of it's history. I actually purchased it from a public high school back east that has a saltwater aqua-culture facility. I was so impressed by these kids and their knowledge. The "frag" was more like a mini colony when I received it. I'd also purchased phytoplankton from the school. I spoke with a couple of the students on the phone about phyto...explaining to them that I wanted it for my clam. Their depth of knowledge, when they started rattling off different species of phyto, and why some were better than others, just amazed me. Anyway, here's my current favorite.
I frag this coral about once every 3-4 weeks just to keep it from growing out of the water...lol. I recently fragged some large pieces off and put them in my frag tank under 250W MHs 10K. (My main tank is lit with 6 T5s.) Here's a pic of the color difference. Needless to say, I was amazed, as this color changed in 2 days.