Most action adventure shows that deal with power start out as: Main Villain > Everyone Else
The Main Hero usually starts with little to no skill and works their way up the power ladder by defeating lesser opponents so by the end it's: Main Hero barely > Main Villain > Everyone Else.
Main tagonists aside and disregarding the fact that whoever wins depends on whatever the writers of the show decide, who wins a fight depends on their skill, environment, and how they use those things to their advantage. DBZ has corrupted a lot of minds into thinking the one with the greater power level always wins.
Yes, Katara and Toph and Zuko all know special moves that only a select few know, but does that mean those little tricks will guarantee victory against every bender they meet? And not to mention, there's no way any of them know every single technique for their elements. People say Toph can beat Bumi simply because she can do something he can't, but I'm willing to bet his 100 years of experience have taught him a few moves she doesn't even know about. Plus, which one lasted longer against Aang?