Stay away from caimans as someone suggested. I personally don't agree with keeping them as captive pets - unless you are extremely experienced. And by that I mean someone with 20+ yrs dealing eth reptiles and specifically Crocodilian species. Someone like myself or example, who has 10+ yrs dealing with snakes and some lizard species has no business taking on a caiman. They're cute looking when they're little but even the dwarf species grow to be way too large for the average person to care for it properly.
I would also advise against a Chameleon unless your have previous experience with them. They're very sensitive to living conditions that are anything but ideal. Prone to illness and stress - not for an average or inexperienced herper.
I just bought a Brazilian rainbow boa and she's beautiful. They have iridescent coloring that sparkles with all the colors of the rainbow when positioned under proper lighting. They're pretty nippy when little but from what I've heard, with proper handling and care they should calm down with age. My little girl is still only a few months old though. They do also require very humid environments but IMO they are worth all the time and money and care it takes I keep them in an optimal environment.
If they don't tickle your fancy then maybe look into a really cool morph of a more common species, for example the snow morph boa constrictor or a blue eyed Lucy ball python or a mystic potion ball.
Also the woma pythons are cool.
If you want a really cool snake for observation and you're ok with not handling it much or at least very minimally then go for an emerald tree boa or a green tree python - again though experience is a must for these guys. If you have experience with cham's and you really want a show stopper go with a Nosy faly panther cham - they have the most amazing colorings you will ever see on a reptile. Like I said though - lots of experience is needed as these guys are very difficult to keep in perfect conditions in addition to keeping them totally stress free.
Good luck with whatever you choose