If you could get me a 2500 gallon tomorrow, I'd set up a rio sarstun biotope.
Thorichthys helleri, thorichthys aureus, chuco godmanni, vieja melanura, theraps irregularis, rocio octofasciata, cryptoheros spilurus.
I don't have dream fish per se, because "dream" by my definition would imply inaccessibility, and an urge to get that individual fish in particular. My favorite fish- amatitlania sp. "honduran red point", aphyosemion maculatum, and luxilus cerasinus are all pretty accessible to me, as I have the HRPs, have seen a. maculatum on Aquabid, and can drive about 3-4 hours to catch l. cerasinus in the wild. Further down the list, I currently have a pair of f. fallax on the way, tuic regularly gets atrato type umbees, and Wes always has Fogo/azul bass. I moreso have dream biotopes, such as sarstun, sucio, lari (have already), tapajos, atabapo, and so on.
Edit: tldr, I forgot a. "altoflava", t. asfraci, and c. "thyrorus" existed. Those are my dream fish