I have had Oscars, aros, clown loaches, puffers and pacu as previous posters have mentioned, and all I have thoroughly enjoyed. Especially Pacu and Oscars.
But one fish I am currently enjoying immensely that hasn't been mentioned yet are my Geophagus Tapajos. I keep them in a group of 7, and they are very personable fish. The more I watch them I can see the different personalities each one has (I guess this is may just be normal cichlid type behavior, but without the aggression.) They are interesting to watch interact in a group, and as individuals who sometimes like their own time and space and have their own individual quirky behavior. They don't scare easily and are not timid fish (with me at least there is no timid behavior, but they are suspicious of new people, like if I have someone else around, they don't get as excited and tend to stay back more. Which makes me think they may be able to recognize their keepers?).
They are always excited for food and splash at the surface when they see me open my door and walk in my apt from across the room. They are very observant in the world outside their tank.
When my arms are in the tank, or Im moving stuff around/doing maintenance they never hide but always come to see what I am doing, often brushing my fingers and hands ( occasionally sucking on my fingers and arms when they have the opportunity.) I am amazed at how trusting these fish are.
When I add new fish, I like their curiosity and how they follow the new additions around just staring at them.