grade A flowerhorn, whatever kind appeals most to you...or go with something else with color, if its only one fish in a 180....also, id make sure whatever I choose was show quality, and not likely to be duplicated elsewhere...
However, if it was me with my current fish, then my veil tail oscar would be in there, as it got me back into fish keeping, and therefore means alot to me.
Also, with a 1000 dollar cap, im going to have to rethink this later, and try to come up with something oddball hopefully.
Personally I think a single guppy, or something of similar size would be hilarious.