i choose #2
it’s funny that everyone else for the most part chose the 2nd most boring logo.
i think #2 creates an organic, moving, molten feel. that mixed with the fact that FE stands for iron could give you a flowing, creative, always growing kind of effect to the logo.
i agree with some that there might need to be another object in there just to make the letters fit together that much more. perhaps it’s a stroke around the letters that matches the shape of the letters or something.
i think you’re off to a good start with any of those logos except the last one. cause that’s just two letters next to each other that aren’t touching or reflecting each other in any way.