Tough one, as there are so many. I don't have any right now, but Melanotaenia fluviatilis or splendida have to rank at the top for activity and color IMO. I'm also partial to most all plecos.
DOWNsized!? But why!? lol I understand, I've been there but I don't think I've ever gotten to less than 2 tanks since like 3 months after I got my first fish lol. 72x18x22? Love that size! Will never have one though, my house can't support it. Will you be going planted or bare or somewhere in the middle?