Wow where to start, they are all my favorite.
Salt water reef because there is so much going on, it is a true Eco system. Everything in the tank is in there for a reason, the fish are so diverse. You can sit there and watch any of a thousand things for hours.
Fresh water planted gives life to a room, they are relaxing. The shrimp, while not having personalities like saltwater shrimp, they are so fun to watch. And the schools of fish are incredibly peaceful
Saltwater fish only are frigging cool, the fish have personalities like none other, and the shapes and colors are unreal. You have aggression but it's manageable.
Freshwater aggressive America's, they have sheer aggression, and that is FUN. They are mean as crap
Freshwater African are great both lakes have their benefits whether it be large numbers and colors with Malawi or crazy shapes and wicked looks like tanganyikan.
All I can say is I dunno because I have had everything except brackish and I love every last one of them equally