First of all, I hope I never do. I love crystals in all their variety, colors, inclusions, weight, and even the variety within each type of crystal. Add to that the carvings and varied shapes... well, I think you get the picture. I don't think I could choose just one.
But, since I've asked the question, I'll see if I can answer it.
If I could choose only one, I would lean on functionality and go with a crystal that can be used for all chakras. That narrows the field a bit, but leaves a lot of options. Clear crystals typically benefit all chakras. We're not talking about just Clear Quartz. There's Icelandic Spar (Clear Calcite), Clear Topaz, and Lemurian Quartz to name a few clear ones.
For those that are not clear, there's Sichuan Quartz, Serpentine, Seraphinite, Scapolite, Rutilated Quartz, Stibnite, Magnetite, and more.
But the questions is, if I had to pick just one.... I would choose Icelandic Spar (Clear Calcite, also called Optical Calcite). My experience with this one is intense. This is not to say that wouldn't change my mind. If I come across a crystal that is particularly powerful for me, I'll use it in meditations and healing often. That's my answer: Icelandic Spar... for now.
So, how about you? Which crystal would you choose if there could be only ONE? What's a favorite of yours, tell us and tell us why?