Goldfish. They take a bit more work in terms of water quality (they're supercharged poop-machines) but they have a lot more personality, variety and are better able to handle the surprise occurences that come with living in a fallible world. Just don't mix longbodies (commons, comets and shubunkins) with shortbodies because the longs are better swimmers and will out-eat the shorts to starvation.
For tanks, you should go by the general rule of 20 gallons for the first fish and an additional ten for every other. You can get away with a little less when they're babies but be prepared to upgrade since they grow FAST. Also, you don't need a full time heater but you should have one on hand. Goldfish (especially shorts) are prone to digestive issues like constipation, and raising the water temp a few degrees temporarily is one of the best ways to fix this.
They do best with large-grain gravel as a substrate. Naturally they're scavengers and you'll often see them "cleaning" the gravel, essentially scraping it with their mouths to eat any leftover food and poop on it. If they're in a sand tank there's a very good chance they'll eat a good bit of sand, eventually resulting in bowel impaction and very often a fatal rupture. Another thing you never hear about beforehand is that you should be prepared to perform emergency "dental surgery" when they get a piece of gravel stuck in their mouth that won't come out. All you need to do is remove the fish, put him in a shallow container full of water (like a betta cup) and carefully remove it with tweezers. It's a lot more nerve-wracking than it is dangerous.