Geryi or Diamond Rhom.... Geryi are just awesome looking, but hard to come by... Diamond rhoms are pretty sweet looking as well and both of mine are a lot more aggressive and swim a lot more than my geryi... the diamond up in my room is still for sale... could work you a good deal man!
Tough Question and depends on what size tank you have available.
I would choose a Geryi as a single specimen fish, but if you don't like/ can't get a big Rhom is a must. else the best P I have other than those 2 is a Compressus(most active and agressive Serra I have).
I think it depends on the fish itself not the species. You would'nt even know that I have a rhom in my tank he hides during the day in a cave , The only way I see him is if I turn the lights on and catch him out in the open then within a few seconds its back to hiding. The only action you see is the guppies that I throw in there with him. His size is probobly almost 4 inches now.
i say go based on you $$$and tank size... if u have lots of $$$ and want a single fish i say go with a Manny or diamond rhom. .... if u want a shoal then go with a geryi or piraya or caribe.
If u are going for something smaller. (smaller tank size and such)... go with an eigmanni or banditi or something like that.
I like my compressus. Real aggressive and very active. My ex girlfriend plays with him all the time. She'l bite at him through the glass and he'll bite back and then he gets all wild. Plus they stay small which is a plus if you live in an apartment like me.