Well the title is pretty much self-explanatory, what does overpowered means to you? How one mod can be overpowered while the others mod that provides the same functionality is balanced?
Like some people says that MiscPeripherals is OP, but I don't think so, or even computer in general (not turtles)
EDIT: Because some people have a huge dumped list of OP mods, so I adjusted the rules a bit:
The format is like this:
Mod X is overpowered because of Y and Y is overpowered because of Z
Example: ComputerCraft Turtle is overpowered because it can make free mining and free mining is overpowered because of making other miner obsolete