Steam!! Which one us problematic because there us so much to like. If we're talking passenger power then a NYC Niagara and a PRT T1 top my list. If we're talking freight haulers I like the classic look of 1920s power with Elesco FW heaters on the front of the smokebox with double airpumps on the pilot deck. The NYC L2a Mohawk is a classic example, but their are similar examples from other roads.
Too many options to narrow to just one. I like the potato chip analogy...