One of my biggest regrets is not researching before buying fish. Back before I moved to Canada, my family had a 10 gallon I believe, and we did almost 0 research, and barely cared for the fish aside from feeding. Now, I regret that, and bought many fish that were way too big for their tank. I have a few stories
Coolest one by far: Black Ghost Knife Fish - these guys were insanely cool, but they didn't last long, and alas, they were aggressive. However, the temp wasn't a problem due to my living in the tropics at the time.
Another quite cool one: A baby Paroon Shark - it swam around for a while and actually survived for like a month, but it too succumbed to my poor management.
Extremely aggressive: Rainbow shark - This one actually survived, but while it was in my tank it picked off a few fish. We ended up giving it back to the reject tank at our local LFS
More aggressive: Cichlids - We got a few cichlids from the tank, but I was always sus on them because they always had their own tank, wherever we went. Turns out my suspicions were right, and they killed off a few tetras. We ended up handing them off to anyone who would buy them for 5 bucks.
Longest lived: Upside down catfish - I had one of these in my tank, and brother had one in his. These guys actually survived the entire time we had them, and mine only died later when I gave it to one of my friends when we moved. They were always cool, and at first I was afraid it would die any moment, but soon I figured out that was the point. Point is, they survived in my brother's 3 - 5 gallon for a few years, so that's a record.
Another long lived one: Some skeletal fish with whiskers - I'm not exactly sure what this one was, and I forgot what we bought it as, but I remember we called it the skeleton fish. I'll attach a picture. It lived as long as the catfish, and was also one of the longest lived ones we had.
Now I'm just listing off my most memorable fish. Also, I have a few mystery fish that I'm not sure what they are, so if someone could id them, that would be great.