Definitely has to be when my 4 year old gets excited about fish and asks me if he can go look at the fish and the corals with his kiddie magnifying glass. Love getting him to care about our seas and oceans.
Best part: I have something interesting and relaxing to watch while the 3 year old is sitting in my lap watching Sprout TV.
Worst part: Hit to the pride meter when something goes wrong with the tank (Fish fights, water parameters/conditions, finding the 3 year old in the tank up to his shoulders trying to pet the Gouramis again by tempting them with more food, etc)
Best: Spawning one of the rare/extinct/endangered species, fry in general, most times they're more entertaining than TV, of course I do have the benefit of several tanks and many species so I light up which continent I'd like to watch LOL
Worst: there's many things - one of the ones I dislike the most is getting a bad shipment where the shipper doesn't take any pride in how the fish are packed or how they may arrive on the other end.
Best: when you can sit back and see the beauty that you created, and then thinking of something you could do better or different. the learning part of it too, like 5 months ago i didnt know what cycling was
Worst: dead fish, watching your fish suffer from a desiase rather than thrive in your tank.