I'll bite-
Rarest- maybe the globifers I had. I paid $45 each, a friend bought some parsons on the same order for $75. Obviously this was years ago before madagascar restricted them. Globifers were really cool looking too- Large thick heads and bark-like texture on their sides. I haven't seen them available in many years now. Maybe a tie with those were brookesia perarmata. Another super cool looking chameleon- I dont' think those are available any more either. I got a pretty decent size group of a dozen or so individuals but failed miserably with them even though I succeeded with theili. I was able to reproduce most species I tried but failed miserably with perarmata for some reason. Maybe I got a bad batch.
Most beautiful- my first pair of veileds. At that time they were rare and just entering the market and expensive- I don't remember how much but hundreds of dollars each for cb babies. My pair flew delta on a plane (only way the breeder would ship such rare lizards) and I picked them up at the airport 45 minutes away. In the months that followed as they grew I thought the male was so beautiful with his gorgeous coloration and high casque. I can't believe how cheap they are now (along with lots of other lizards) and how they are now considered sort of unglamorous.
Of course, I've thought all the species I've kept are beautiful, that's why I chose them, but those first veileds were magical to me.
How about this category- most underappreciated you've kept?
I would say Oustalets. They are way cool in person compared to photos. Heavily built and textured- really cool warty skin and high contrast coloring around each scale-especially on their heads so their is a lot of "pop" to their scalation, plus nice spikes down the back and a thick head and the males get huge! Plus a really nice personality. Really prehistoric, dragonlike. Kind of makes smooth species like panthers seem kind of wimpy. Would probably eat panthers for lunch given the chance. They aren't brilliantly colored, but their coloration and contrast on the scales makes them really neat. Hardy like veileds and panthers too. These days similar in price with veileds for florida caught. They really deserve more popularity.