African (Malawis, Tangs)
African river (planted, with a large shoal of Hemigrammopetersius caudalis and some Pelvicahromis taeniatus)
Amazon
Reef (if you can afford it)
Asian Blackwater
ADA
Iwagumi
Personally I think monster fish are very boring, because I've seen them before and they don't really do much. They're just very BIG. And I don't really know of any monster fish that have lovely colours. They all seem to be silver or muddy brown or grey.
I would go for the Africans or a large shoal of colourful fish like Paracheirodon axelrodi, Hemigammopetersius caudalis or Trigonopoma pauciperforata and a few softwater cichlids and some fancy catfish.
If you can afford it though I think a reef tank would be spectacular, with some Dwarf Angelfish and lots of small colourful Wrasses, Damselfishes and Cardinalfish.