Would you like 1 fish or say a community of oddballs?
A thread in the predator section recently came up about hi-fin wolffish/rainbow wolffish (Erythrinus erythrinus). Would be a good candidate for my money, tankmates are iffy, I've heard of it being done but I wouldn't personally do it. Your descision may also boil down to do you want one fish that reaches around 8" in a tank of that size? They are more active than the common wolffish (sleeping logs) and do look nicer in my opinion. Erys got a mean set of jaws on them aswell, they look the business.
Another possibility are snakeheads, they come in different sizes, so you could in theory either have one or say a breeding pair. For your tank I would say a snakehead of up to 12" or so should be OK.
So possibilities include:
Bleheri
Gachua
Stewarti
Punctata
Lucia
(I think these are the most common ones sold that might be ok for your tank - if you're interested in these I would read up on each specific requirements).
<a href="http
/snakeheads.co.uk/" target="_blank">http/snakeheads.co.uk/</a>
Worth a browse if they interest you.
Another candidate for an unusual tank would be puffer fish, these are normally kept solitary (though some types can be kept in groups). Though some suffer from laziness and you may want a more active tank, puffers like the arrowhead and congo are nice they are in the 'lurker' category meaning they spend most of their time hiding and become active at feeding. Though if these unusual fish interest you it might be worth reading more on them.
The Puffer forum is a good site to read up on all the types of puffers and what they need.
Is a good site, go to the PufferPedia (Under the learn tab) to see all varieties.
Pirhana might be a good road to go down if you want something to capture people's imagination (though be warned they are not the maneating, no fear machines that people believe them to be, often they are skittish). I think a good choice for your tank would be a Serrasalmus elongatus, they're mean and definately a solo occupant. There are other types of pirhana availabe, probably the most common being red bellied, people recommend keeping them in groups so I think your tank may be on the small side personally.
Though if you want a community of oddballs then the list is quite long, fish that should mix ok with others if you choose tankmates wisely will be things like:
certain polypterus (senegal is the most common)
reedfish
ctenopoma acutirostre
African butterfly fish
Smaller eels eg. peackcok
Dwarf girraffe catfish
EDIT: Can't believe I forgot this one, exodons. They are like mini piranha (what people imagine piranha to be like). They shoal together well and are very active. They need to be kept in large groups though, over 12 I would say as otherwise they pick on the weakest until only a few are left and even then you can be left with 1.
I think it can be narrowed down to get the fish you want by the question asked at start of my post, would you want one fish or a group of fish?
i agree although i have to admit i have been very tempted at times to move my male betta in my 55 gallon once its set up. i can just see him now, swimming around, enjoying all that space to himself 
and the fewer water changes.... ya. 
i would love it if they made a filter system, thats affordable, where you never had to do water changes. maybe in another decade. 
