If I had to choose between the brightest, most objective obnoxiously colored ones that are pretty much always that color as long as they're happy, I'd have to go with (in no particular order)
Thorichthys maculipinnis
Rubricatochromis exsul
Aulonocara stuartgranti
Trichromis salvini
Enantiopus melanogenys
Fossochromis rostratus
Gymnogeophagus rhabdotus
Mesoheros festae
Amatitlania nigrofasciata Rio los almendros, captive strain
Herichthys carpintis, captive strain
Parachromis multifasciatus
I won't get into a lot of my more subjective honorable mentions just yet... there's too many, but they're all pretty conditional (certain time of year, breeding coloration, etc). You could argue that that applies to some of these as well, but I think generally people will still be impressed by them if they saw them in your house. Some that come to mind however:
Gymnogeophagus rhabdotus
Cichlasoma dimerus
Pundamilia nyererei
Amatitlania myrnae
Amatitlania altoflava
Amatitlania siquia "Guanacaste province"
Amatitlania septemfasciata "Rio San Juan"
Nandopsis haitiensis
Kronoheros cf. umbriferum "Rio atrato"
Cribroheros rostratus
I ran out of file uploads for this message but Cribroheros robertsoni, Cribroheros altifrons, Apistogramma agassizii alenquer, Pelvicachromis kribensis proper, and of course alenquer red discus are also up there.
