Spring of drowned Virtuous Man.. I can't actually remember if this is a real one, or just shows up in fanfics on and off, but it'd be a pretty neat political tool. If people could verify that it worked, require all elected officials to be dunked and locked.. or perhaps, make a dunking+locking a pre-requisite to run.
I believe that one was from an anime episode where Taro was trying to use it to make Happosai so virtuous that he would choose a non-embarrassing name for him. He had the wrong water, though. There was also a victim of spring of drowned priest in either the anime or one of the ova's.
Depending on how exactly you interpret them, the Yamasenken and Umisenken are game breakers by themselves. Both of these schools likely contained more moves than we saw in the anime/manga, and Ranma could likely develop new techniques based off of them, but even with only the canon skills these schools are limited primarily by the conscience and honor of the people who know them.
I've seen a couple of fanfics use the idea of Ranma learning to generate hot ki in one hand and cold ki in the other then spin himself like an ice skater to create the spiral, thus making a powerful wind technique that is limited only by how much power he has to put into it. By using the modification of the Hiryu Hou Toppa, it becomes a very focused and dangerous attack. While this is stretching the canon, it is based firmly on the principles laid out for the techniques.
If Ranma actually mastered all of the techniques from the series and was willing to use the dangerous ones, he would be immensely overpowered. Only a small percentage of the techniques such as Ukyou's flour bombs require that he carry the necessary ingredients with him, but if he mastered hidden weapons, that wouldn't be much of an issue.
Hidden Weapons by itself could make Ranma the most skilled smuggler in the history of the world, or the most feared terrorist. Being able to carry nuclear or biological weapons in a completely undetectable manner has many nasty possibilities.